I think you are right that Marathon is not Adventure material, but I also doubt that Catacomb is an Adventure book: beyond not wanting to break Perkins, they have also cited that most groups were not processing two books of adventure material in a full year. So, I forsee them pulling back on Adventure specific material this year.
Speaking to this specifically, from what I have heard they have brought in plenty of freelance writers to help with a book this year, plus they just hired another staff member in Kate Welch. Plus Adam Lee (one of the Story Designers for D&D that works with Chris Perkins) said he was put in charge of an upcoming book.
Seems more than likely that all of that means they needed help with 2 APs released closely together. The reason I have guessed in the passed that Broadway and Catacomb are Waterdeep and Undermountain are because of the release dates. Being so close together means that if Catacomb and Broadway are both APs they have to be linked, and I can easily see them being Waterdeep and Undermountain books meant to compliment each other.
That also fixes the "Groups cannot play through it fast enough" issue, by having both books be tied to one Adventure, with one detailing the city and the other detailing the dungeon. Seems more than logical to me. But as you said, we shall see soon.