Absolutely not. But I would like to read about them in my local newspaper, or - in a more specialized case - an Italian gastronomy guide for connaisseurs. The fact that things are not all the same, are what makes it worth subscribing to them. Back to the web and communities, linking an quoting are valuable of course. But they are another use case, different than sharing a topic between forums and viewing the thread in context, maybe participate directly.
You are right that in some cases you don’t want to mix identities and certainly not completely merge dispersed communities altogether. But you may selectively merge only those parts where it makes sense. Be it on a topic-by-topic basis, certain (sub)categories, tags or specific groups of people sharing content.
It is not really about ActivityPub either. The protocol is quite low-level, building on top of HTTP. You can build anything with it. Very often when mentioning ActivityPub, people automatically tend to think of microblogging (Mastodon) as that is the most popular application until now. If you consider this domain, everyone is sort of creating their own ‘unique community’ with it, defined by who they follow. This creates their personal timeline. Other than that they may have chosen to create their account on e.g. mastodon.technology and their server timeline loosely has the theme “technology” to it. But this domain does not really fit Discourse, of course. It is microblogging, not community forums after all.
Currently some microblogging applications are extending their domain with the concept of Groups. You might see this as a community concept that extends across the server boundary. So while I’m on mastodon.technology I might subscribe to the “spaghetti” group, and “climate change”. But it is still just microblogging → everything is compressed and flattened into my timelines.
What is a succesful community? What are its bounderies, what is inside and outside? These may be very clearly defined, and relate to identity and diversity. One thing they do not relate to per se, are specific server boundaries!
Though I went very broad in Community has no boundary, it is thinking without these artificial server boundaries I addressed (and how that may increase quality, quantity and activity of the community. I did not go into identity, which is a good point to also address). Thanks for responding there @Mevo, I’ll reply in due time.