Meta may be many times bigger, but that does not mean they are not interested in killing all competition. That is just the megalomaniac mind set.

Not sure a buy-in option exists with the Fediverse.

Meta may not even care if they get partially de-federated. They can still claim they are part of the Fediverse with a simpler start-up. People who were considering trying the Fediverse may think they already have with their Threads account.