The US is the largest country with english speakers, but it does not have the majority of speakers [1] (just India plus Pakistan have as many). So your assumption would be wrong, even based on statistics.
That's why I qualified it with "native" - the US has by far the most English-as-a-first-language people. In my head everyone speaks like an Anglo guy from Idaho, and since HN is text-only, metaphorically speaking the lights are out, so I'm left without a clue where other people are sitting or where they are from.
In other words, if you found yourself in a dark room with a thousand English speakers who all sounded like Jesse Pinkman, what country would you assume you were in?
I'd be willing to bet that less than half of this site's users are American. There is a huge software engineering community in Europe, for example, and there are more Europeans than Americans, let alone the large populations of Asia, Australia, New Zealand...
English is the dominant second language in the world.
I would agree with that, at least for this site. I wouldn't be surprised if the ratio was 1 to 10, since tech skews it significantly. For example, I work in tech in the US, and the only American I've spoken to today was my summer intern, and - oh hold up, I just typed that and realized she's from Canada. OK, I've been in work meetings all day and I have not spoken to a "US American" during that time. I suspect that other sites, particularly some subreddits, are mostly American, though.
1. Most native English speakers are in the US, so the accidental assumption that someone is American is more often than not correct.
2. The internal voice that reads text to me has a generic male American accent.