Let me go over this again for you.
When you joined lemmy.world, you agreed to their ToS. I have not joined lemmy.world, therefore their ToS does not apply to me. They owe me nothing, and cannot delete my account nor any of my users from lemmy.zip. they can ban my users from lemmy.world, remove their posts etc, but they're only doing that to their copies of the posts. The original copies are on lemmy.zip and lemmy.worlds actions do not affect any other instances that has a copy of the lemmy.zip original.
Therefore they do not provide my users with a service. If lemmy.world shut down tomorrow, lemmy.zip users would still have service while lemmy.world users would not.
Similarly a website i have never been to might have a ToS, but I have not agreed to that ToS, therefore it cannot apply to me. Said website is not providing me a service.
So we've established who is providing who a service in this scenario, which is lemm.ee providing a service to their user. That user isn't using lemmy.world, therefore isn't receiving a service and isn't beholden to their ToS.
Lemmy.world have banned that user from their website because the user is saying their under 18. But they claim to have done this because in their ToS they say they don't provide a service to under 18s. But that user has not agreed to the ToS.
While lemmy.world is entitled to do whatever they want imo, it's their website, to say it's because of their ToS is incorrectly applying it. They aren't providing a service to the user. Lemm.ee is.
Again, they can do whatever they want, it's their website, but its not how it applies to lemmy.zip. If I was to enact that policy, it would be under the code of conduct as that is what is applied to moderation of remote instances.