IANAL but I think it's more complicated than that...IIRC if you want to do.business in the EU, you need to comply. Not sure if.you can argue that HN does 'business' though

HN and YC does do buisness, amongst other things display and rent out advertisement space that is both displayed to, and on behalf od eu citizens and companies?

Categorily false. GDPR applies based on your users location, not the website/buisness.

Please consider not stating things as facts that you havnt bothered to research.

And you are stating theories as facts. If GDPR holds on American businesses with no legal presence in the EU, then Chinese censorship laws apply to us companies outside China. Or do they?

Makes no sense at all and the analogy is really bad. GDPR holds on US businesses only for the EU users. If they don't have EU users they don't have to care about the GDPR. US businesses can have different set of rules for the US and EU market (that's what Facebook plans to do about the GDPR). The GDPR doesn't apply for the US market and the US users. Also US companies can limit their service to the US market if they don't agree with the GDPR.

And how do you suppose they would enforce this law on a US only company? An unenforceable law is no law at all.

Most likely the enforcement would be applied as a restriction on any future business done in the EU assuming the US company refuses to pay the fine.