I think I don't really understand the benefit of data portability in the situation.
Twitter was my home on the web for almost 15 years when it got taken over by a ... - well you know the story. At the time I wished I could have taken my identity, my posts, my likes, and my entire social graph over to a compatible app that was run by decent people. Instead, I had to start completely new. But with ATProto, you can do exactly that - someone else can just fork the entire app, and you can keep your identity, your posts, your likes, your social graph. It all just transfers over, as long as the other app is using the same ATProto lexicon (so it's basically the same kind of app).