I guess the fear (and probable strategy for Meta) is to first establish themselves as just a reliable instance with a closed app (Threads). From there, it's a slow crawl to bring in the users, from outside but also from other instances. They have multiple tools for this: the infinite budget to develop Threads with exclusive features, just a better app, maybe influencer friendly ad models. The list is infinite.

So where's the rub? Meta is just introducing activity pub to more users.

The problem is two step: They'll eventually will lock in the platform from rest of the fediverse. It'll might be years from now but it'll happen (unless it's killed first if course). This hurts rest of the fediverse by making it smaller: They will hook in users that would've otherwise chosen another instance and now are in Meta's side fence which has turned into a wall.

Note: Not an expert, I just like to speculate.