I think there's a certain subset of people that like to read HN but not submit/comment, either for reasons of anonymity or just because they don't want to. They may be inclined to comment from time to time, and create a temp account to do so, since HN does not allow anonymity. Also, it's possible they
had an account that was banned/shadow banned, and refuse to make another full account out of protest.
That is, this may not be someone using a throwaway account in lieu of their real account, but because they don't have a real account to use.