It is easy to maintain freedom of speech on Lemmy. The idea that your instance can defederate from instances like Lemmy.ml, but everyone on Lemmy.ml can continue to post on their own server, and federate with any other given server by default is the entire point of federation and proof of concept that the fediverse is great for free speech. You’re under the impression that all Lemmy servers are supposed to be part of one big whole, but that’s not what the fediverse is for.
You have to understand that “free speech” is a negative freedom and it doesn’t mean everyone has to listen to you. It just means nobody can force you not to speak.
This is exactly what the “Taxation is theft” morons don’t understand. They think if the government no longer takes their cut, everybody will just have X amount of money more, and the market won’t just swallow that up without giving you a single thing in return.