My understanding was that GDPR requirements apply to all citizens of the EU regardless of where the company is located. Someone else can chime in if that's not accurate!
It doesn't care about anyone's citizenship, but it can apply to US-based entities if they're sufficiently targeting Europe in offering goods or services.
But is HN targeting Europe or any geographic area? If someone from Europe decides to participate, isn't that a conscious decision to participate in a forum outside of Europe and thus outside of the GDPR? Should someone expect their own, local laws to apply around the world?
The last time I touched on this with our legal team (and it was not related to GDPR, it was just related to data storage of EU Citizens) the storage location was a very relevant key point.
Having said that, what about CDN's? I'm certain that lots of comment data is located in CDN's across the EU, US, and Asia.