when mastodon first started, activitypub didn't exist yet. the fediverse was a bunch of GNU Social instances communicating over the OStatus protocol. these instances were well-established, and they did not take kindly to the popularity of Mastodon and all the new users taking over their existing, quiet culture
the existing, quiet culture, by the way, was a bunch of channer shit and blatantly anti-gay and anti-trans memes. the instance we saw most often on the federated timeline in those days was shitposter.club, a place which virtually every respectable instance now has blocked. mastodon didn't really *have* blocks back then