I don't like Meta, I don't like Facebook and I don't like anything Zuckerberg does, including him. But I think that we're overreacting and creating assumptions that perhaps won't be true.
Everyone is free to de-federate or not to de-federate from that project. But let's think for a minute. ActivityPub projects are open source, that means that everyone can read the code of a specific project, being Mastodon or Lemmy. That won't change, it's not so easy, luckily for us.
Can they add on features that aren't part of the spec? Sure, they can, but everyone is free to add those features to their projects, it's not mandatory. Their project won't be another Mastodon instance, or another Misskey instance, but an independent project inside ActivityPub. It's like saying that Pleroma is Mastodon, that's not true.
Can they cut ActivityPub support? Well, it's their problem, ActivityPub is alive right now without their support, and we're doing pretty well.
Will they mine everyone's data along the way? That's the most serious concern for me too. But unless we look at their code, we don't know yet.