ActivityPub isn't aiming to be a standard that "bridges" interoperability across all existing instances without any work. It's a standard that people will either have to port to or bridge to. It's heaviliy informed by other standards, and we worked hard to get feedback from other groups... a number of decisions in ActivityPub happened due to feedback from talking to Friendica and Diaspora devs, for instance. And of course we've been working with linked data people.

ActivityPub is well informed, but it isn't magic pixie dust where it automatically makes interop happen. Recently I've been studying a lot of lisp history; that's also a place where many languages diverged. There is no magical route. Common Lisp was an effort to try to bring interoperability amongst the various lisps, and I think is probably the most successful language interop effort of all time; in that case, much of the code that existed did work with realtively little porting, but code did need to be ported. The good news is though, you can write lisp code today that applies to a large number of lisp implementations which have adapted the Common Lisp language... and those are the only languages in which lisp interoperability is easy. But of course there are a lot of lisps which don't do that, and even my favorite (scheme) is not common lisp interoperable, and barely interoperable between its own implementations.

Will ActivityPub be the Common Lisp of the federation world? It would be great if it were. We could even build tools that will allow interop to be easier through bridging. But the best routes will happen due to porting. We've tried to be as informed as possible by all the federation implementations out there, but it hasn't been easy... even getting people to review and be part of the process was a large portion of what I did early on. But we studied all the major standards that were out there.

I'm not sure what answer you're looking for though. I don't know what result we will get. If ActivityPub could be to federation standards to what Common Lisp was to lisps in the 1980s, there may be hope for the federated web. I don't have a crystal ball to know... all I have is our efforts.