Ad Hominem means against the person. It's very poor form to attack your opponent's character as the implication is that you can't attack his argument.
Andrew Montford complains that this critical appraisal of a recent controversial paper is mostly ad-hom , but he is tantalisingly non-specific. It seems that his gripe is to note motivations as ad-hominem insisting "the motivations are irrelevant to the science." Debatable, since Roy Spencer (the author of the now discredited skeptic paper) is Chairman of the George C. Marshall Institute a motivation which may conflict with the dispassionate search for truth that should be scientific enquiry.
So, having widened the definition of ad-hominem to include discussion of motivations how long does it take Montford to go back to criticizing his opponents motivations? Not long , the folowing day Monty snipes at scientific journal Nature, and delivers a whole blogpost in which he rants (an unsupported assertion) that 2500 IPCC scientists are "corrupt, so bereft of any integrity."
Would it be bad form to wonder perhaps Andrew Montford's motivations are less than a dispassionate search for truth?