- ActivityPub is an open protocol. If Meta goes all-in on it, they'll be implementing a transparent spec everyone knows. Modifying that would send obvious shockwaves through the network and signal their non-cooperation. There isn't a covert way for them to really try this.
- Mastodon itself is AGPL licensed, meaning any Meta fork (for whatever reason) would be subject to "provide the source code of the modified version running there to the users of that server. Therefore, public use of a modified version, on a publicly accessible server, gives the public access to the source code of the modified version."[0]
- Meta has no reason to. If they decide the app is sufficiently popular without ActivityPub integration, then things return to the status-quo for Mastodon. Meta loses what little control they had over the direction of the standard/protocol/applications and nothing really changes.