The @Mastodon and #Fediverse confusion is partly due to the fact that Mastodon only supports the Server-Server aspect of #ActivityPub while using its own #API for Client-Server interactions. Net effect, it has the lions share of clients generally perceived as Fediverse clients -- which is a strategic point of vulnerability over the longterm, IMHO.
@kidehen @Mastodon @tallship @Gargron @peepstein @blaine @atomicpoet Interesting point. If the "Fediverse" is about federation then the S2S aspect of #ActivityPub enables that federation, not the AP C2S part (which doesn't even require a federated server). For that reason, I think clients using the Mastodon C2S API are actually Fediverse clients (among others). If you'd like a challenging exercise, try defining the full Mastodon C2S API using the AP C2S spec (including event streaming, etc.)