It seems that you believe that just because Lemmy is federated

No, that’s not what I’m saying. I’m saying that because federation is a two-way street and can ben enabled or disabled between any two instances, it will always be possible to have instances that are unaffected by threads. Certainly, some instances will choose not to federate with threads - and so there will always be the possibility to join a community (or create one) that shuts itself off from that influence. Take beehaw.org for example - they didn’t like the influence lemmy.world had on their community so they defederated. They are no longer influenced by lemmy.world. The point I am trying to make is that because of that fact (that possibility), there is no reason at all to desire to limit the total scope (userbase) of all of lemmy. If you want a private/closed community, then make one or join one - you don’t need to suppress everyone just to achieve your own personal goals for a community.