the idea is to allow decentralisation …
so the forum does not rely on or need to be controlled by one single site organisation,
and can outlive the sites currently carrying it, and scale well beyond just what hardware one single site might be able to afford, and be very hard for anyone to censor.
think of something like usenet without the spam that is more flexible and open
re: federated profiles .. and auth across sites ,.. already exists in federated socnets
eg check out RedMatrix - they have one-click auth working between sites
The identity stuff in that sounds somewhat like what you describe - (though more like some kind of crypto key than a usename underneath and apparently not tied to a site’s domain either - so a user can even move their account to another server without losing their connections)
and for delivering posts that and also Diaspora, Friendica, Pump.io, GNUSocial and many others already have lots of goodies working..
but what they all seem to lack are forums that actually behave how I’d like for public forums !
it would be nice to be able to to connect together the best of both worlds!