Now that I think on it a bit more, yeah, I'm going to take a guess that the remaining Archdevils get stat blocks in this one as members of the "30 high level foes" club. Since it seems pretty certain that the adventure takes place at least in part in the Nine Hells, it would seem likely they would be directly involved (and Asmodeus would certainly want to counter a threat like Vecna appears to be in the adventure), making this the perfect place to finally update them. I'm also wondering if Primus and the remaining upper level heirarch modrons will get stat blocks as well - maybe we'll get a callback to the Great Modron March module, or that Vecna's plan involves messing with the machinery of the multiverse directly in Mechanus. On the more certain side, I'm sure Kas (and, obviously, Vecna) will get statted out in the book.
EDIT: Oh, didn't they say they had Bane, Bhaal, and Myrkul statted out for Descent to Avernus, but had to cut them (both the stats and their extended storyline) for room? Well, this seems to be the perfect place to bring them back in, especially since Bhaal's symbol is on the alt cover (perhaps doing double duty symbolizing both him and Avernus!)