Interestingly, I originally believed HN is pure-upvote based, then I learned it from other HN comments that says karma affects ranking and I believed it.
So I'd say it's just an unsubstantiated rumor/misinformation getting circulated in the comment section from time to time, combined with the impression of HN having a "magic algorithm", so many believed it without any fact-checking. Also, the quick-moving nature of HN comments somehow created a confirmation bias that makes the idea appeared to be true.