@blacklight
I didn't read the spec, but from what I heard from first opinions, they want to make user own their identity. In ActivityPub your identity belongs to server you're using. So when you're migrating between the nodes, you're dependent on the will of both servers to say "yeah, that's still him".

Currently Fediverse is like using custodial Bitcoin wallets without having ability to store your keys yourself. You can move from one service to another, but you can't really be sure that your identity is yours.

And it's a problem I saw in the Fediverse long before I heard about BlueSky.
Don't misunderstand me. I'm not very sympathetic to what they're doing. But from what I heard (and I could've hear wrong) they solved one of most important AP problems.