You know how Apple has extended SMS with iMessage? Like that.

In other words, they take something open and established like activitypub, and then build all sorts of cool features on top of it, but those features impose lock-in.

Eg. Maybe they make it so there's some way of attaching media directly to posts, but only if the post is both posted and viewed from a Meta instance. And then, in a few years once they've become dominant due to everyone switching over to their platform out of fomo of those features, they break compatibility with activitypub and ruin the underlying structure of the fediverse.