This is not the case.
The Lemmy devs decided to make federation either whitelist oriented or blacklist oriented by an admin setting. Blacklist oriented means that your server will federate with all servers it hears of except ones the admins block, whitelist is the opposite, the admin has to federate on purpose.
Since Lemmy is developed by and widely used by whiny tittybaby communists who plug their ears any time someone says something they don’t chant to themselves daily already, whitelist based federation is the most common way the other instances do it. As a result, most of the other servers know about this one.
@mister_monster@monero.town verifying