Thanks for asking the question and putting so much care into stating it.

I have no idea what kind of methods HN employs. I personally always check the posting history when I'm in doubt about the intentions of a poster. Most of the time, I find an extensive amount of fairly well considered comments on a variety of topics. That leads me to the conclusion that the account is 'genuine'.

To state it a bit more naively: if a Sybil attack would require the attacker to craft so many constructive comments in order to evade detection, it could actually have a net positive effect on the community as a whole.