It’s not using activityPub but isn’t the principle idea kinda the same? Revolt is open source and you can host your own server, and then each user can decide to which one to subscribe to using the client.

The servers are not talking to each other per se (I don’t really see the need for that anyway in software like this), but your client will look a lot like a fediverse once you populate it with dozens of different instances of revolt. At least that was my understanding of it, correct me if im wrong.