Your point would be true if ActivityPub was a properly defined specification, but it isn't. It is voluntarily vague in many places to accommodate for the differences in implementations, which means that each implementation needs to understand how other software works to interoperate. So in practice it is more accurate to say "Protocol like Mastodon", because even if you can talk with Mastodon it doesn't mean you can talk with Pleroma or Pixelfeed or others.

More details here: https://overengineer.dev/blog/2019/01/13/activitypub-final-t...