I agree that fediverse is only a few million. But the fediverse is also highly populated with refugees from twitter and reddit at the moment, who just want another stable and popular social platform similar to what they’ve always used.
If anything, the fediverse will have people with stronger opinions: either they’re willing to change social media because of a couple bad changes (and aren’t too attached to the fediverse), or they’re hardcore fediverse fans who are less likely to move to threads than your average twitter user.
If we assume it to be a 50% split, then meta has a chance at stealing half the fediverse by promising a larger user base, thus more content, but on the false premise that Threads will be backwards compatible with the fediverse forever.