It uses its magic crystal ball, while simultaneously consulting a legion of captive demons to determine this and other similarly unknowable information.

Unknowable? It's can check your browser footprint and ask Facebook/Google if you are on holiday.

I suppose you have to use your TOS for that. (In fact, a banner that tells European users that they aren't allowed to use your site is probably the easiest way to insulate yourself -- if you collect their data because they used your service illegally, I'm not sure you can be blamed.)

Violating TOS is not illegal...

You misunderstand the GDPR if you think you carry its protection around when you visit non-European locales.

Just because a piece of paper in a European bureaucrat's filing cabinet says one thing doesn't make it reality

It doesn't. It just checks that somebody is connecting from an IP address geolocated in Europe.

I tried to be sarcastic, but I think my English is not good enough :-)

I'm a native English speaker and I picked up on your sarcasm. You did great :-)

That said, a constant source of miscommunication from native english to native english that is written, is missing sarcasm. Just a side effect of not having non-verbal communication cues.