Free speech is not an “inalienable” concept. To be honest, the adjective “inalienable” doesn’t well describe a “concept”, more “right” or a “privilege”. Nobody can take away a concept from you, only the exercise or realisation of that concept.
That said, the Lemmy dev’s code of conduct is not a binding document, and given their political views, I wouldn’t even bother opening it if I was adminning my own instance. And nobody owes you your freedom of speech - if an instance decides to enforce the rule “no talking about politics on Wednesdays”, nobody has any right to anything else. Preference, sure, but right? No.