Hacker News can comply with GDPR today and choose not to comply the next day. They forfeit nothing in the process of selectively abandoning GDPR, in regards to being governed by US law and not EU law.
If my company is located in the US and only governed by US jurisdiction (eg I do not operate in the EU in any manner), there's no such concept as admitting the EU has jurisdiction over my company and granting the EU new global powers (such that I can provide the EU jurisdictional reach into the US so that it overrules or competes with US law). That isn't my decision to make if I operate inside the US jurisdiction. The US solely decides jurisdiction within its zone of political control. That is, the US has the final say legally in all regards in that case. I could choose to voluntarily comply with GDPR, however the EU has zero power to force me to. I can flip flop back and forth a thousand times, or not, it makes no difference.
If you ever find yourself wondering about these concepts, change the scenario to China. Under what scenario does the EU or US have power to dictate laws within China? None. It provides a perfect clarification every time. Nobody could possibly be confused about who is in charge of law within China.