Yeah its predecessor, OStatus, was pretty much that: you start with a feed of anything, then you add another feed for the reader to comment, then a way to send the comment back to the source so everyone can see it, and lo and behold you've got yourself a social network brewing. Activitypub is a rethinking of the whole thing with clear requirements and specs instead of some informal pieces added here and there, which is a good thing. Except all the existing RSS feeds can't be used as such.