Your criterion for distinguishing an ad hominem argument is not at all valid.
It's enough for someone to say "that theory is the darling of every cannibalistic child-molesting moron in Sweden". If they said "that theory was created by a cannibalistic child-molesting moron and thus it's wrong", all they would be doing is giving away the irrationality of the implied ad hominem argument. It's much more effective and much more common for an arguer to toss insults and let the reader's emotional reactions create the conclusion.
Your criterion for distinguishing an ad hominem argument is not at all valid.
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An ad hominem argument, also known as argumentum ad hominem (Latin: "argument toward the person" or "argument against the person") is an argument which links the validity of a premise to an irrelevant characteristic or belief of the person advocating the premise.
There is a difference between saying "cannibalistic child-molesting morons agree with that" and saying "thus it is false." That's why we have the term ad hominem: to distinguish between valid statements (i.e. "child molesters believe that the sky is blue and that encryption technology should be legal,") and invalid statements ("Thus, the sky is red and PGP should be banned.").
You are complaining about something legitimate, but what you're complaining about is not ad hominem. The closest term is probably "rhetoric."
If you read the definition you posted, you'll notice it says "linking". The link can be explicit or implicit. The original article certainly established an implicit link between the beliefs of the left and the belief in global warming.
This would be a valid critism of the article if you weren't taking that quote out of context. The author was pointing out how heavily politicized the argument it is and why that should make us carefull about picking sides. He's very explicit in pointing out that motives of a scientist does not necessarily make him right or wrong. He also mentions that climate-change skeptics have their ulterior motives as well.
I should mention that I don't necessarily agree that the original article only sneaks in ad hominem attacks but I would say that gratuitous insults towards "Marxists" on the left in certainly mar the conclusions it aims for.
In generally, I actually think the article tries to make the valid point that science shouldn't be manipulated by anyone. On the other hand, I think the point that since massively increasing emissions of all variety of pollutants could very plausibly be causing irreparable harm to the environment, we should cut them back seriously. I don't think that line of reasoning makes me a Marxist. I won't pretend this is a scientific argument however. But they are many situations where people have to make decisions where the science isn't settled (and waiting till the science is settled is just the decision to do nothing).
The unfortunate thing is that the level of national dialog has kind-of degenerated to the point where you get traction by clothing yourself in the authority of science and belittling your opponents. Some part of the article might serve as an effort to improve this dialog. The part where the author talks about the "Western left-intelligentsia, their kitbags stuffed with all the sub-Marxist and ethno-masochist flapdoodle of the modern Academy. They hate capitalism, they hate Western civilization, and they hate their own ancestors."
Your criterion for distinguishing an ad hominem argument is not at all valid.
It's enough for someone to say "that theory is the darling of every cannibalistic child-molesting moron in Sweden". If they said "that theory was created by a cannibalistic child-molesting moron and thus it's wrong", all they would be doing is giving away the irrationality of the implied ad hominem argument. It's much more effective and much more common for an arguer to toss insults and let the reader's emotional reactions create the conclusion.