Your comment reads better without the Ad Hominem. You have an opinion, it differs from his, that’s fine, and that’s all you need to say.
Summary: Ad Hominems can be factually correct yet they are still Ad Hominems and still the lowest form of disagreement.
And my point in this case isn’t that the entire argument was an Ad Hominem, but rather that it was a (possibly correct) Ad Hominem followed by a perfectly reasonable point. I stand by my assertion that the point is stronger on its own without the Ad Hominem.
This assumes that the OP's only goal was to debate. I opine that his secondary goal was to be a little cheeky and possibly evoke a chuckle, which was successfully achieved with me! You can interpret the "curmudgeon" comment as an ad hominem to the extent that his exclusive concern was the logical presentation of his argument, which I do not think is true for most cases in human discourse, on or off the internet.
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.
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."