@IgnisIncendio

The only forum-like part of a server is its Local timeline. It's the only thing that requires you to be on that server or looking at that server's website.

So, whether you regard a server as a forum depends on whether you use the Local timeline or not.

I personally don't use Local much, so to me the Fediverse is entirely cross-server.

But if someone does use Local a lot then they might feel differently.