activitypub at its core is incredibly simple afaik… its just a set of defined APIs and that follow ActivityStream schemas
the actual user identifiers we interact with in software aren’t the same as what activitypub uses internally (they’re all URIs so look more like https://<instance>/<path>/<to>/<user>)
it wouldn’t be difficult to use a similar mechanism as they’re using right now to direct a particular domain or subdomain to a user ID: it’s all rewritten anyway!