Worst case, with Lemmy, some server op decides to rage quit and shut down the server, and you're left in the same situation as if HN decided to just shut down without notice. In the other scenario, with Lemmy, you do the same thing you would if the same thing happened at HN: either play along or take your ball and go home.
You're right that being federated doesn't magically make people into reasonable humans. But, at least with Lemmy, you have an easy option to start up your own server if you don't like how someone is running one that you participate on.
FWIW, in the context of the fediverse where there are reportedly instances run by actual neo-Nazis, it's both wrong and confusing to label someone a fascist for instituting a policy designed to keep those people out. You can disagree with the policy, but it's particular important to choose accurate labels here.
Allowing communities to decide for themselves: Great.
Banning anyone who wants to use your supposedly open source & decentralized software from using certain words that you deem inappropriate in American English is not acceptable. Spanish people are not allowed to use the word for the color black, for example.
"Fascism" is correctly used to describe a set of far-right ultranationalistic and militaristic ideologies that are directly responsible for the deaths of millions of people. If you're going to use that label, I want to at least see overt racism if not actual calls for violence.
This example is particularly egregious because the brand of authoritarianism we're looking at is antifa, which quite literally defines itself in opposition to fascism. The far right does not have a monopoly on authoritarianism, and using the word "fascist" to describe left-wing authoritarianism gives up rhetorical ground that you don't want to sacrifice.
And what you are saying is a stretch.
Or is that your intention?
So that cat is kinda out of the bag.