Thank you. Ad hominem has nothing to do with talking about the credibility of the author. Rather, the fallacy occurs when you rely on that information to challenge the position.
What is this if not challenging the arguments the author is making? tptacek claims this article shouldn't be on HN because the author is somehow unqualified to share his opinion and therefore it shouldn't be of interest to anyone.
'Well, I don't agree or disagree with what you're saying, I just think YOU shouldn't be able to say it' doesn't seem to be different from an ad hominem attack in practice in this case. It's refusing to engage with the actual content on quality or validity grounds and instead engaging with some unrelated point.
The original post is not arguing 'you should agree with what I'm saying because I'M saying it' and it's also not saying 'these are facts because I say they are'.
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.
