People have articulated all kinds of actual harms, including two possibilities in the OP, but frankly they're irrelevant.
No, they didn't. The harm listed was that Meta will make a shinier platform that will syphon away users, that is happening regardless and is not a harm that is a result of federation, it's a harm that's a result of meta having more money to build a better platform.
We know what Meta's goals are, and we know they have absolutely no moral standards whatsoever. Exactly how they try to accomplish those goals doesn't matter. We shouldn't give them the opportunity to try anything.
There goal is to launch a twitter competitor with a lot of users and make money off advertising. Nothing about that conflicts with the fediverse.
Like I said, this thread is filled with a bunch of people shaking in their boots about the company who must not be named rather than actually providing sober rational assessment of what's likely to happen.