“basic web tech stack” is quite a bit hand-wavy, don’t you think?
Yeah, indieweb sites could do a lot of things: authn and authz, content syndication, backlinking, Social Graph, etc. But none of them had been standardized and put into one single protocol. ActivityPub is precisely this protocol.
Saying that “we can do that with a basic web stack” is not that different from saying “we can have a protocol to publish XML content without styling using a basic web stack, why do we need RSS or Atom?”