You’re getting it a bit backwards.

Here’s the simple answer - can I delete your account or prevent you from accessing your home server? No. Therefore I’m not offering nor providing YOU a service. I am providing a service to people who have signed up to lemmy.zip.

I own and run lemmy.zip. I state that only over 18s can sign up. I do this because lemmy as a software cannot prevent a person from seeing nsfw content if they’re under 18, which is a limitation of the software. However admins can turn off nsfw content entirely, such as feddit.uk does. Anything tagged nsfw their users cannot see.

The home instance is liable for their own policies. If they allow under 18s and have nsfw turned on, that’s their risk not mine.

A NSFW post on one server can federate to lemmy.zip and then federate to a server hosting a minor (i.e., the minor passively used the lemmy.zip website)

That’s not how it works and they’re not using lemmy.zip at any point during that anyway. Otherwise your argument would suggest that we are all subject to every websites ToS even if we’ve never been on that website, which isn’t the case.