So i may have understood wrong here but i think the analogy is like saying something like:
instead of reddit being a central hub with “rooms” (subreddits) branching off, we instead have multiple reddit hubs which have their own sets of rooms. But a lot of those rooms have little back doors or hallways to the same room at the other reddits, so the users in a particular room can see eveything in all the other alternate version of the same room as if it was the same one.
Like being in the same subreddit without realising its a compilation of contant from all the other rooms across the lemmyverse.
But also that if the people in a particular room start getting rowdy or dangerous then the other rooms can lock their backdoor connecting the rooms so the unruly users cant come in from that other room until they decide to unlock the door again.