This appears to be the explanation:
https://bsky.app/profile/did:plc:w4xbfzo7kqfes5zb7r6qv3rw/post/3mjmztqnuwc2g
In Bluesky, the PDS talks to the relay talks to the appview goes to the client. Blacksky set up all four last year. But they only deployed their PDS and client, at first. They used Bluesky's relay and appview. This wasn't clearly disclosed. Then there was a censorship scare, and they switched to their own appview. But apparently they're still using Bluesky's relay. This wasn't clearly disclosed. Now relay death kills Blacksky.