I mean, it is just one bad actor.

If you look at it in these terms, you understand that Facebook has an interest in making sure that ActivityPub doesn't too large without Facebook having a say in it.

I don't think that ActivityPub is having any present difficulty keeping itself niche without Facebook's help - fedi has a total active user base of something like 2million, it's very literally a rounding error on Meta's user numbers. If there's a battle here, Facebook is already winning.