You can host your personal data, but as long as Bluesky Social, PBC is running the main Relay and AppView, and there's no easy way for Relays to talk to each other (like e.g. it works in Mastodon), they are in reality a centralized service.

Oh yes, and they are the only ones owning and developing AT Protocol, which makes it much more a currently-open-sourced protocol rather than a standard that is jointly developed by the industry.