Earlier quoted context omitted.
Bluesky is a Twitter-like platform like Mastodon, so the comparison makes sense if you look at the primary implementations. SSB is also decentralised, while ATProto is federated - like ActivityPub.
User Identity in ATProto is decentralized, it's meant to use W3C DIDs. That's actually one of the things that bugs me about ActivityPub... unless I'm running my own one-single-user instance I won't have control over my own identity. It's also very weird how even on a supposedly "federated" system the only way to ensure you can access content from all instances (even if they differ in philosophy or are on opposite sid…
The W3C spec leaves all the hard parts to vendors, which is why the only DID implementation up to now has been Microsoft's, which relies on an AD server in Azure. Much decentralise.
Bluesky's isn't that, but a hash of some sort, which is centrally decentralised ... on their servers? I think this is one of the bits of AT that isn't finished yet.
But "W3C DID" is not a usable spec in itself, it's a sketch at best.