Why did google implement XMPP originally for Google Chat? They originally did federate with XMPP servers. But eventually, they decided that federating wasn’t worth it. At the time they still used XMPP, they just refused to federate.
Why wouldn’t you use ActivityPub as a protocol if it has been proven out and does 90% of what you need? Just because they use ActivityPub, I don’t see them ever Federating in any meaningful way. Essentially, I think the protocol is well designed and Meta is just using it as their starting point with no reason to share any data with the fediverse.