I think this is incorrect - federation depends on a “true” version. A community is tied to an instance, that’s where the true version is. When you’re logged into another instance and you comment in that community, your comment gets copied to the true version, as do comments from other federated instances, and all of them get copied from the true version to a version on yours.

When one instance defederates with another, that copying stops happening. If a copy of that thread is still on your instance, and you comment on it, your comment won’t get copied to the true version, so it also won’t get copied to other instances; only users on your instance will see it.

That’s my understanding, but I’ve only been here a few weeks.