Personally, I think we've already seen proof of just how easy the endgame is now, but I've never done the Kael fight, so maybe this is even more of a feat. Dissonance on Alleria finished off Kael'thas Sunstrider (the 25-man version, not the 5-man fight in Magisters' Terrace) with just twelve people. Looks like kiting was a big technique, but otherwise, just seems like they were able to overpower everything else in the fight. They actually tried it with 10, they say (they dropped Al'ar, Void Reaver, and Solarian with 10 the week before), but the MCs in Phase 4 were a bit much, so they pulled in two more.
This is a Sunwell guild, so they've got pretty much the best gear you can walk around with in the game, and as we've said here before, these instances really got nerfed twice: once when Blizzard nerfed the actual bosses' HP and damage, and a second time when Blizz gave us all the 51-point talents before the expansion.
But it's definitely an achievement, toppling a 25-man boss with less than half the required group. Oh, how the mighty have fallen, sure, but don't fret, raiders seeking a real challenge -- we're sure Blizzard will make up the difficulty when we all step into the new endgame in Northrend.