I'm expecting FB to make some sort of proprietary fork of ActivityPub. I'm not quite sure what the point of Threads is, from Zuck's POV, excluding the desire to eat Musks's lunch, which is huge, so that's more than enough. The man needs a win, bad, after that Metaverse flop heard round the world.
But everything you'd consider an advantage to the Fediverse is a downside to his business model, and the things it enables, like user-controlled hosting, don't suit his ends. Maybe he intends to colonize this place, too, but up until very recently there wasn't a meaningful user base to gain control over. Still isn't. Mastodon got the biggest boost, they're up to 8 mil users now, a healthy number for sure, but Instagram is pushing a billion users, so the number isn't much to him.
So he's not going to actually like anything about the Fediverse, or what it offers, nor will he like its downsides much. ActivityPub, as it is, doesn't do him any favors. I can't imagine anything other than the skeleton of ActivityPub getting repurposed into something else that runs Threads in the locked down way that Meta is accustomed to. He just seems to be grabbing at anything that's loose for the taking.