One Legion in all universes is the dumbest part of WoD. It means KJ and Magtheridon went and tried to do the exact same plan that had already failed, knowing that it had failed, to invade an Azeroth that was more prepared than it had been before.
Also Metzen has written plenty of other stupid shit before, the community fucking hated him when he was still at Blizzard. Heck, the way people in this thread are talking about cosmic stuff sounds exactly like people losing their shit at spaceship protoss Draenei back in 2005. Never mind how characters like Illidan and Kael'thas were ran through the mud for raid content.