No, it’s not. In a discussion about the credibility of an academic researcher, their track record is entirely relevant. The same isn’t true of an HN comment.

"Valid ad hominem arguments occur in informal logic, where the person making the argument relies on arguments from authority such as testimony, expertise..."

https://en.m.wikipedia.org/wiki/Ad_hominem

It appears that the original comment was valid ad hominem, but still ad hominem.

Any ad hominem cannot inform you about the quality of an argument. It _can_ inform you that you should look at an argument critically but, even in the "valid" form, it doesn't address the argument.

Slight of hand. We were not discussing the credibility of an academic researcher, we were to discuss the contents and logical merit of their submission.

Both were ad-hominem. The first one also suffers from a pompous bias: What does it even matter if the submission is not peer-reviewed? It could have been posted on a blog! It is dropping little seeds of doubt, to weaken and attack something you don't want to attack directly:

"A warning to everyone that the linked website does not adhere to Wikipedia guidelines for being a credible source."

Expect better. If you demand peer-review, then review the contents as a peer! You have the opportunity to get what you want, instead of needlessly attacking a fellow scientist' credibility.

> Both were ad-hominem

Surely a distinction can be made between attacking someone's character and attacking their claim to authority. Within every scientific paper there's an implicit claim to authority and here the track record has weight alongside whatever qualifications they may have.

It's "sleight", from the middle English for "skill", not "slight".