I've been reading up on #Bluesky, and it isn't what I thought. As far as I can tell (please correct me if you know otherwise), it an open source federated social media platform.
It runs on a protocol called #ATProtocol, as opposed to #ActivityPub (#Mastodon and friends).
Other Fedi protocols include #Diaspora, DFRN (#Friendica), Zot (#Hubzilla), or Nomad (#Streams).
However, bridge services are needed for Bluesky/Mastodon federation, and there's mostly just one big Bluesky instance.