"Grandparent absolutely intends his insult, in proximity to his argument, to undermine his disputant's credibility."
I don't think that that is clear at all. In fact, his derision reads more like a thesis statement to me: "X is a curmudgeon. Why? Well, because of statements like Y."
The first thing the grandparent says is that Dave is a curmudgeon. It's absolutely an ad-hominem, more specifically it is the poisoning the well[1] variety. Everything read after the opening sentence is tainted with the idea that Dave's actions are that of a curmudgeon. Most ad-hominem's are accidental; they're so prevalent in internet discussions that it's difficult not to engage in it, so I'm sure the grandparent meant no harm. He had a genuinely good point, but I agree that it's somewhat ruined by the well poisoning.
I don't think that that is clear at all. In fact, his derision reads more like a thesis statement to me: "X is a curmudgeon. Why? Well, because of statements like Y."