Because a fediverse is any group of technologies that talk to each other via a common protocol. In 2025 that’s ActivityPub and has been for awhile. It would be one hell of a stretch to assert that a single platform with its own home made protocol that doesn’t talk to any other technology in the entire fediverse as part of that fediverse. So at best you can say Bluesky has its own fediverse. And if one fediverse is going to be “the fediverse” it’s going to be the one that actually connects all the most common platforms people use today, including Diaspora.