It's a subtle difference, but one probably based on opinion. tptacek doesn't make any claims on the arguments themselves, but rather the placement on HN. So it'

It's a subtle difference, but one probably based on opinion. tptacek doesn't make any claims on the arguments themselves, but rather the placement on HN. So it's not ad hominem, but it is censorship. His claim is more along the lines that only "pre-approved" sources should be worthy of attention, not people with low credibility. But he's not challenging the arguments themselves. So, no logical fallacy, but maybe elitism or something. I would further argue that the HN voting system can be relied on to determine what is interesting instead of credibility. Certainly, I believe just because 37Signals or Dustin Caldwell posts something doesn't make it interesting, but the voting seems to disagree with me. At any rate, we have an existing system to determine what should be relevant comment, so there's no need to invoke credibility here.