The Fediverse is not one enclosed, unified entity under one centralised rule.
It’s a common misconception that “the Fediverse” is a network platform created by whomever, usually Eugen Rochko. And Mastodon, Lemmy, Misskey, Friendica, Pixelfed, PeerTube etc. are Web UIs for the Fediverse, and Mona, IceCubes, Tusky, Fedilab etc. are mobile UIs for the Fediverse which mimic the functionality of certain Web UIs.
This is complete non-sense. None of this is true.
Instead, the Fediverse is a patchwork of many different things that work together by speaking common languages. And with “work together”, I mean “work together ever so barely” in many cases. Mastodon and Lemmy are not different clients for the same server thing. They connect, but they can hardly interact.
These “apps” aren’t client apps. They’re server applications. They provide a whole slew of very very different server backends.
There is no “Fediverse suite of apps” either. Just about everything in the Fediverse is developed and offered separately from one another.
Mastodon, in particular, ignores the whole rest of the Fediverse and tries to present itself to its users and Fediverse novices as “the Fediverse”. And when Mastodon users discover that Mastodon is, in fact, not “the Fediverse”, Mastodon makes them believe that everything that doesn’t work exactly like Mastodon is broken.
Oh, and no, Eugen Rochko didn’t invent the Fediverse. Evan Prodromou did. In 2008. That was when he took his recently launched Twitter alternative Identi.ca, open-sourced its technology under the name Laconi.ca (later StatusNet, now part of GNU social) and laid its protocol open under the name OpenMicroBlogging (now OStatus).
The Fediverse consisting of multiple different kinds of interacting servers came to exist in 2010 when Mike Macgirvin launched his Facebook alternative named Mistpark (now Friendica). He built it on top of a whole new protocol, but he gave it the ability to speak OpenMicroBlogging as well, thus connecting it to StatusNet. One key feature of Friendica is still to be able to connect to everything that moves and then some.
Mastodon was built on top of OStatus, too. But the intention was not to connect it to already-existing StatusNet, Friendica, Hubzilla (a much more powerful Friendica fork by Mike Macgirvin himself) and Pleroma (which had started out as an alternative UI for StatusNet). The idea was rather that using an already existing protocol was easier for a young and barely experienced coder than designing an all-new protocol from scratch. Mastodon never intended to be interoperable with anything else.
Even when Mastodon introduced ActivityPub as early as September, 2017, it was not to be able to interact with Hubzilla which had it first, two months earlier. By the way, ActivityPub is another one of Evan Prodromou’s creations, but this time, he wasn’t alone.
The idea behind Lemmy seemed to be similar: Build a Reddit clone, but without the hassle of designing a brand-new communication protocol. The difference was that Mastodon was already quite well-known when Lemmy was launched. When Mastodon was launched, StatusNet was considered dead after its only really known instance, Identi.ca had switched from OStatus to pump.io. As for Friendica, Hubzilla and Pleroma, nobody knew they existed, much less that they spoke OStatus. OStatus was there, ready to use, but to most people who came across it, it felt unused. So I guess that when Eugen Rochko created Mastodon, he unironically and sincerely believed that he was now the only one using this protocol, nobody else ever would again, and Mastodon would only ever connect to itself. Mastodon’s whole very concept is to be a “federated walled garden”, decentralised on the inside, but not letting anything else connect.