It kind of makes me think of how odd it would have been if many of the old forums named themselves like bookclub.phpbulletin.com, metalheads.vbulletin.net, or something.
There’s nothing wrong with doing that, obviously, but it’s struck me as another interesting quirk of fediverse instances/sites. Generally as soon as you visit them you can tell by the site interface or an icon somewhere what software they’re using.
Activitypub is the protocol. Mastadon/Lemmy is the API. Just because two applications share a protocol doesn’t mean they can talk to eachother.
The Spotify app uses the HTTP protocol, like your browser does, but that doesn’t mean you can view any webpage from the Spotify app. Idk if activitypub is actually a protocol, but it is at least analogous.
While it makes sense for apps/networks of a similar type to interoperate (like microblogging), apps/networks of different types may not make sense to integrate.
So for servers on the same network/app, it makes sense to include it in the server name, so that people know what app/network it belongs to.