Would have to agree here, logic does not really have that firm a place in WoW. The sotry has always been a firm adherent of teh cartoony and camp, which means that if Bolvar wants to attack with an army of undead dragons with necromancers flying them, then it can. It does not have to spend all htat long trying to explain how Bolvar found all these dragons to ressurect, or how the necromancers know how to ride the dragons, it can just have fun.
That being said, if Thunder bluff turns out to be a ruse, and Sylvanas took that time ot attack somewhere else, then that is a perfectly fine explanation of how the raid came to be.
We will see though, i don't think anyone anticipated the Dazar'alor raid outside the few that theorized it might be a mini-raid from the datamined maps.