Clearly, they got their lore from somewhere.
Even more clearly, there were night elves in what is now called the Eastern Kingdoms both before and after the Sundering. While we know the ancestors of the modern Blood Elves were Highborne, culturally distinct from the kaldorei but physically identical to them, these elves were not so distinct. They had ancient ruins (like the ones found in Seradane and in Duskwood, patrolled by the Green Dragonflight and eventually usurped by the Dragons of Nightmare) and preserved some of the kaldorei culture, even including their druidic traditions. (Remember, before Malfurion Stormrage, Cenarius taught others, although laughably few night elves sought him out as arcane magic came to dominate their society.)
Whether these Eastern Kingdom kaldorei were stranded there by the Sundering or even migrated there later (Gaedrin Moonfang's journal seems to indicate at least a few kaldorei made their way across the sea), it's clear that their presence left traditions behind, but we see no evidence of any living night elves to have passed them on. Furthermore, since we know that the ancestors of the blood elves found themselves changing when separated from the kaldorei moonwells and had to create the Sunwell to compensate, it's possible that these Eastern Kingdoms night elves would too have slowly changed, lacking the immortality of Ysera's bargain with Malfurion and without access to the greatest knowledge and power of their rebuilding culture. No moonwells, no priesthood of Elune, a few scattered ruins and potentially a whole host of insane feral druids imprisoned beneath trees.
Where did the non-worgen night elves of these sites go? One possible explanation is that they didn't go anywhere. We know that the high elven descendants of the Highborne kaldorei can breed with humans. Perhaps night elves can as well? We have Arator the Redeemer and the children of Rhonin and Vereesa Windrunner to attest to elf/human interfertility. (And apparently, we also know Windrunner sisters are warm for human forms.) Over thousands of years, a few night elves not asleep under magical trees for their falling into savagery could easily be absorbed into the larger, more prolific, much shorter-lived human population in this way. And this would mean that whatever scraps of lore they managed to pass on could be transmitted down the years to their almost completely human descendants as "the old ways" Celestine of the Harvest mentions.
We don't know that this is what happened, of course, but it doesn't have to be. Even if the kaldorei were stranded in places like the Twilight Grove or Seradane, or migrated to them, they could have made contact with the then rising humans in a limited fashion and passed on some of their druidic secrets to them before lapsing into the same kind of endless sleep other druids would find in various barrow dens, or even dying out if their immortality was severed as that of the Highborne in Quel'Thalas.
The Scythe bites its wielders
Now, we next have to move on to considering Goldrinn's Fang, or the Scythe of Elune as it is more commonly known by the modern mortal races. We know that the ancient Goldrinn died during the War of the Ancients, before the Sundering, fighting against the Burning Legion. We know this war took place all across the ancient world, and that at its end, the single continent of the world was torn into at least three: Kalimdor, Northrend and the Eastern Kingdoms. We know that at some point after that disaster, night elf druids came north to the Grizzly Hills and attempted to create a new World Tree there, a disastrous attempt that pierced the confinement of Yogg-Saron and had to be destroyed.
What is interesting about this is, why would druids do this? Especially since, at this time, pretty much every druid in the world had to spend a lot of time sleeping in order to help protect the world tree they already had. When they weren't sleeping, they were serving under Malfurion Stormrage, who didn't seem at all inclined to leave Kalimdor or order anyone else to do so, much less to go there and start making a world tree when he already had a perfectly good one. Later, the furbolg would attempt to use the fruit of the ruined tree to raise the dead ancient Ursoc and succeeded, only to find him tainted with the essence of Yogg-Saron. But where did they even get the idea that the fruit of a world tree could raise an ancient?
This leads me to posit that the world tree in Grizzly Hills was created by druids of the pack. They had the motive: Goldrinn, their inspiration and object of veneration, was slain during the War of the Ancients. They had the time, as they were exiled, unable to join their fellows in their constant vigil over Nordrassil. They had the knowledge to at least attempt it, as many of them were trained directly by Malfurion himself (Gaedrin calls Malfurion "my former Shan'do" and writes as through he was often directly lectured by him). And furthermore, if the world tree Vordrassil was a botched attempt by the druids of the pack to raise Goldrinn, it explains in part why the Wolf Cult under Arugal so successfully took root there.
When Arthas had his San'layn raise Arugal's shade from the dead and transplanted it to the Grizzly Hills, it's possible he did so for more than simply some kind of obsessive desire to collect a pack of werewolves to complete his "classic monsters of moviedom" assortment. Sure, adding werewolves to his vampires and zombies and liches has a certain cachet, but let's imagine that there was more at work here. Why would Arthas assume the human trappers of the Grizzly Hills might make receptive members of the Wolf Cult? Did he know or suspect of the origins of the failed world tree? The Lich King was very powerful, able to send his will ranging out across the land and had access to all the accumulated knowledge of his subjugated nerubian slaves, long residents of the frozen north. He had access to Kel'Thuzad, a former member of the Kirin Tor (who have long been interested in Silverpine, home of Arugal and the place he first summoned worgen to) -- and therefore, Arthas would know of the Book of Ur.
The rebellious claws
Furthermore, the Scythe of Elune itself figures into all of this. Its first modern appearance is in Ashenvale (right next to Bough Shadow, an ancient portal to the Emerald Dream exactly like the one in the Twilight Grove), where Velinde Starsong also uses it to summon worgen, at around the same time as Arugal. What are the odds that a night elf sentinel and a Kirin Tor magus, using completely different researches and means, should both summon forth the trapped former druids in bestial humanoid wolf forms at the same time? Both within reach of ancient portals to the Emerald Dream (Seradane, in the Hinterlands, and Bough Shadow)? While Velinde herself is no more a reliable narrator than Ur and his book (the Scythe/Fang may have affected her mind as it did the ancient druids of the pack), it's fascinating to note that she takes the Scythe, travels to the Eastern Kingdoms to find Arugal, and promptly vanishes from history in Duskwood, right near the Twilight Grove and its portal to the Emerald Dream.
Flash forward to the Wolf Cult in the Grizzly Hills. When the Alliance first makes contact with them, a cedar chest containing a mysterious item is required before they'll agree to work with the Alliance forces in the area. This cedar chest most likely contained the Scythe -- but the question is, how did it get from Duskwood to Northrend? And what happened to it after Arugal and his Wolf Cult are defeated? Well, speculate on the following.
Heirophant Thayreen, the night elf druid who sends you to discover and cleanse the source of Vordrassil's corruption, very specifically says, "My ancestors once planted a tree in the middle of the Grizzly Hills." What if she's not being figurative? What if she's a covert member of the Druids of the Scythe, the group created by Alpha Prime out of the deranged remnants of the druids of the pack, and when you defeat Arugal, she recognizes the Scythe as Goldrinn's Fang and helps guide it to its next destination? What if the Scythe itself seeks out those who can or will make use of it, possibly due to Goldrinn's spirit seeking rebirth but tainted by his death at the hands of the Burning Legion just as his once-noble fellow wolf ancient Omen was?
The reason the Wolf Cult would have taken root in the Grizzly Hills would thereby also be the reason the worgen curse spread so quickly in Silverpine and Gilneas, the same reason the Scythe so quickly raised a force of worgen in Duskwood, the same reason Velinde found herself with more worgen than she'd intended to summon in Ashenvale. These regions were places where the druids of the pack were forced to slumber, where their very nature had infused the land, possibly even where their bloodlines descended through generations. (My ancestors, she says. How many generations back? Not too many, one might assume.) We find renegade druids raising a world tree in Northrend, and worgen flock there. We find an ancient druidic tradition among humans who could never have seen a night elf before they sealed themselves away, and worgen soon arrive. The Scythe of Elune travels from place to place, carried and lost, yet always arrives in these regions just as the worgen spread. Before his return, when Goldrinn's tainted essence is purified of the demonic Lycanthoth, it's possible that the usurper who sought to steal Goldrinn's mantle could have used the Scythe as well, reaching out through it to taint and corrupt even as Goldrinn struggled to guide it to the right hands.
It's impossible to say, of course, but the clawed hand of rebellious druids may well have shaped far, far more than we can see as yet.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.