This is rather interesting, given that we didn't actually know (and still may not know) that the next expansion would take us to level 100 as opposed to level 95, for example. What's more, these items appear, from the dungeon journal at least, to drop from heroic mode raids, as well as normal mode and flexible raiding, but not from LFR. It's possible that these are the brand new rewards that Blizzard were hinting at dropping from Flex and above but not from LFR.
They only appear to drop from from Garrosh himself, but an heirloom that takes you all the way to the next expansion's max level and perhaps beyond has to be an improvement on the Dragon Soul "hey, replace your epic drop off the last boss with a green" syndrome. Certainly, however, many people will be upset that this isn't dropping from LFR.