There are a lot of tanky posts coming from lemmy.ml. Their whole purpose seems to be to troll and spread their bullshit far and wide. They are nearly as bad as the alt-right. They argue in bad faith and celebrate authoritarian oppression. The beehaw mods might want to consider defederating.
‘Fiefdom’ is a domain of a feudal lord, which would not be anarchist, by any definition.
If you simply mean that different groups with different sets of rules and outlooks exist, then sure, but that is abstract enough not to really be specific to the Fediverse; most other social media platforms also have some form of private groups too.