I have a CS and law degree and IMHO the divide here is the CS people tend to believe "ad hominem" has a technical definition beyond, well, the definition
You seem to have missed the operative word there: rather.
Saying that someone's argument is bad so they must be real dumb means you're saying "A (the reasoning attack) implies B (character attack)", not "B (character attack) means I don't have to even bother with A". The latter is an ad hominem, the former is not.
An ad hominem here would be, "jesus I already know this author is so dumb/bad that I don't even have to bother addressing any of their arguments". "This author's reasoning is awful, so clearly they're dumb/bad" is simply not the same thing.