If they wanted to tap into a big user pool they'd link it to Instagram, instead, which has about 2bn MAUs, as opposed to somewhere between 9 and 12 million.

The obvious reasoning for moving to a federated solution would be Meta thinking that it's a cool business opportunity to eventually link all the social media users they ALREADY have. Remember, they own Facebook, Whatsapp, Instagram and Oculus, and at the moment none of them talk to each other, beyond them forcing Oculus users to have a Facebook login, which most of them dislike.

Besides that, BlueSky promotes itself as a separate federated service, which I'm assuming is something they'd like to match if they could when launching their own Twitter alternative.

ActivityPub provides a ready-made path to both of those things they can just... plug into without having to develop it from the ground up, like BlueSky is doing. One has to imagine that the dozen or so million people already using the tech are barely a speedbump along the way, wether they choose to stay linked up to Meta's billions of users or not. Ultimately, open source is open source and Meta can still use the tech to power their Twitter clone and eventually plug it into Instagram and add two billion people to their userbase overnight.