I'm not so sure; he probably didn't know the extent of Aduin's power until he saw it for himself, and Sylvanas is still trying to convince Anduin to join their side. You could be right though; we may unwittingly free him as per usual Blizzard writing. He hadn't captured us though, so it doesn't make much sense to me for him to claim he has what he needs now as we teleport out to Oribos.