ActivityPub, Matrix, XMPP and NNTP all do superficially similar things in very different ways. I personally think of ActivityPub as being a bit like multiplayer RSS; Matrix as being like a replicated pubsub open object database; XMPP like a overgenerously extensible messaging protocol; NNTP like a semi-closed federated BBS.

They all have different primitives and semantics - it’s not just data types. Matrix is the only one which actually implements a full data structure replication protocol between nodes, for instance.

Braid https://braid.news/ is one way you might be able to abstract thag operation on HTTP tho.