Lemmy is not a “free speech” platform, unlike Voat. It can be moderated. Offending instances in the Fediverse can be blocked and all that stuff. As long as the moderators do their job, they can filter everything they want to filter, just like Reddit.
The more interesting question with Lemmy is if the federation will actual have any advantage in the long run, as cutting other instances off is the easiest way to moderate them. Which than in turn means the users have to hop between server, which is annoying and will in turn will lead to more centralization again.
For the time being I see Lemmy not as “The Solution™”, but more as a “not-Reddit”. It can and will run into all the problems as ever other Web forum will.