You're right, but I half agree with the intention. Sure, one could say HN was built in the US, domiciled in the US - but it's still a tech forum on the global internet. As for whether "most" people on HN are from the US, look I can't really prove or disprove you here, I have no statistics, but I'll say that the US is ~1/4th of the english speaking population (330M/1.3B).

Obviously, I'd guess the demographics of Americans is more than a quarter here, but I (think) there are enough non-Americans that you can't really be certain that the individual you're speaking to is actually American.