What I mean is that a subset of all Linux communities agrees on a common set of rules and forms a community of communities. Content of all communities is shared with everyone who subscribes to one of the communities. Every community moderates its own content. If one community decides to have stricter rules than the others it can defederate. Basically just like on the level of instances.
What stops us to just defederate from lemmy.ml is that the community is hosted there and all members are linked to that one point of failure.