Does "the Mastodon protocol" mean ActivityPub?

In the the ReadMe, and the short description at the top of this project's code repo, it says: "a server for the Mastodon protocol"

Mastodon uses the server<> server protocol of ActivityPub, which is a W3C standard, not its own protocol. But it does use a bespoke server<>client protocol, not the AP one. Which one does this project implement? From searching for "ActivityPub" in the issues, it seems like it's AP, in which case it would be good to specify that in the ReadMe and project description.

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