Oh? Admittedly, I haven't seen the MM (I'm not running a game, and I like some surprises!) But I hadn't gotten that impression from the Worlds & Monsters stuff. I'm not sure if I like that; on the other hand, it adds a certain amount of...credibility...to Orcus, et al. In previous editions, it really wasn't obvious why the gods of light didn't just destroy the demon princes/devil lords; they were, after all, not gods.
Now, smiting devil lords involves picking a fight with Asmodeus the god--and smacking a demon prince involves going up against former primordials (who nearly beat the gods in the first place...).