It's fairly specifically _not_ a fork; they've re-implemented atproto (in, what else, Rust). This is important, as there are now two real in-production-use AT Protocol impls (plus a bunch of toy ones).

Sorry I got that from the description they've put on their GitHub repo. Suppose referring to the front-end as a fork of bsky.app. Shrug

Yeah, I think the frontend's just a fork, but that's not really the interesting bit.