Obligatory pedantic reply: the OP would be engaging in ad hominem if he argued that Dave Winer is wrong about real names on g+ because he's a curmudgeon. Instead, he argued that Dave Winer is wrong about real names on g+, and also he's a curmudgeon.
This is an incredibly silly claim that people who enjoy engaging in and supporting ad hominem always make when ad hominem is pointed out. Grandparent absolutely intends his insult, in proximity to his argument, to undermine his disputant's credibility. He doesn't need to literally say, "because he's a curmudgeon, his argument is wrong" for it to be ad hominem.
"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.
My claim is pedantic, but it's not silly. I'm not a fan of either ad hominem or garden-variety insults, as I hope my comment history will demonstrate, but I do think it's worth drawing the distinction on principle.
