jwz has it absolutely right at the end of his post on mastodos/mastodonts on him referring to "Mastodon" instead of “ActivityPub" or "the Fediverse”:
Right now, there is *only* the Mastodon application and Mastodon clients, and those clients speak the Mastodon API, not really ActivityPub. There is only a network of Mastodon servers, the amount of other-ActivityPub happening is negligble, which is super sad but understandable.