One thing I keep coming back to when comparing #ActivityPub and AT Protocol: ActivityPub is basically a convention on top of the web stuff we already have. An actor is a JSON-LD document you GET. An inbox is an endpoint you POST to. If you already run a website, you can bolt this on without changing the site’s basic shape. Ghost didn’t set out to join the fediverse, and then years later it could, just by adding an endpoint.