ActivityPub is cool because we finally have an open protocol to post and subscribe and reply to content across apps.
Mastodon is not Twitter. I get why people think of Mastodon as open source or distributed Twitter, but it’s more than that. If you add ActivityPub support to your WordPress site, others can see the comments and reply from Mastodon! I think this will get more obvious as time goes on and apps like Tumblr start to implement ActivityPub support as well.