There’s no rules for the Fediverse, all it means is that they utilise the ActivityPub protocols to be able to federate with other websites that also use it (there’s others, but basically irrelevant now).
Mastodon requires OAuth2 for apps to get access to your account because it was designed that way, and Lemmy wasn’t, it’s as simple as that. Any platform can be part of the Fediverse (including Reddit, Twitter, Facebook etc if they really wanted to), which also means that platforms can also do anything they want.
Definitely Phantasy Star Online - even today I still play and work on it because it’s just the perfect type of game for myself.