No, you have a fundamental misunderstanding of how federation works.

The user is on lemm.ee. When they make a post, they’re posting to a local copy of the community, and never directly onto the server. Thats not how federation works and so your points are all totally invalid.

At no point does the user ever make contact themselves with that external server. They never physically touch lemmy.world in their actions, everything they do is on lemm.ee, and so lemm.ee’s Terms of Service is the one that applies to the user.

As has already been hashed out multiple times, Lemmy.world’s ToS is not relevant here and does not apply to an external user.

This is totally up for debate, and you don’t understand it. That’s fine, learn from people who do know and take it with some humility.

Oh and if you still don’t get why its important and how wrong you are - if you were right, then every single post you make is federated to thousands of different fedi servers. Suddenly, you would be legally in contract with all those thousands of servers and their ToS. Imagine if I put in mine that users have to pay a small sum of currency for every post that they make on my server. In your incorrect definition, you would be liable for that just like lemmy.world have applied their ToS against someone who isn’t one of their users.

What lemmy.world should do is update their moderation policies/code of conduct/whatever to specifically say that they will ban you if they suspect you are under 18. Its such an easy fix. This never had to be so wrong in the first place.