But these other implementations are on the same ActivityPub protocol, some may have added vocabulary; that's the value of a federated protocol anyone can implement.

You wouldn't call everyone that uses email as a "Gmail user", you're blurring the value of the point of a federated protocol: it's not this gimmick where you have to use this one software, this one platform, in order to talk with anyone else on the same platform (such as with the-app-is-the-service and the-service-is-the-app notion in the world of mobile messaging apps), instead it's where anyone that speaks the same federated protocol is able to intercommunicate without any special translation, bridging, etc--it's all native.