Earlier quoted context omitted.
Ad hominem fallacy fallacy: http://laurencetennant.com/bonds/adhominem.html Pointing out that someone is not trustworthy when considering whether or not to trust their conclusions is not ad-hom.
Attacking a persons 'trustworthiness' instead of dealing with their arguments and evidence is pretty much the dictionary definition of the ad-hominem diversion. It doesn't interest me to learn that he kicks cats or dresses in lingerie and calls himself Marjorie at the weekends. If you believe that he is wrong, then show where and how he is in error.
But, when thinking about whether he's honest or not being given examples of previous dishonesty is relevant.