Well, Lemmy is, when you get down to the technical level, centralized.
Each instance is a centralized unit, with a server owned and run by an individual or a group of people. Each instance replicates and hosts content. Since each instance provides the content directly, they are responsible for the content, same as Twitter is responsible for the content on Twitter, even if the content is a screenshot/copy of a Reddit post.
So I, as a feddit.de user am subject of feddit.de's TOS, since I am legally their customer. feddit.de is responsible to clean illegal content from their instance and from all replications from other instances that they are hosting.
Regular social-media-related law totally applies to Lemmy, with two caveats. Many of these laws have a triviality limit, meaning they won't apply to networks below a certain user count/yearly revenue.
Federation means that each instance is technically a separate social network, so their user count is not added together. And since all Lemmy instances I know are non-profit/non-commercial, there is also no meaningful revenue.
But for laws without these limits (e.g. GDPR) there is no salvation for Lemmy, and once Lemmy becomes big enough for anyone to notice, there will be lawsuits.