Weelll, depends on what you mean with "joining your own server." The model is definitely different from ActivityPub's, but lots of people on the network already have their own PDSs or "personal data servers". They do still go through Bluesky's relay, but nothing is really stopping people from running relays as such, it's just fairly costly (as in some hundreds of dollars per month) currently as they need to hold the full state and history of the network (but apparently that's being worked on.)

But it's definitely a federated protocol even though it's different from AP