Because "I disagree with this" is semantically different from "this is stupid". They communicate different ideas. You can disagree with something without thinking it's stupid. A reader can and probably should react differently to a post being intelligent, but wrong, versus being just plain idiotic.
The post on question also didn't call the author an idiot, just said that the paper was idiocy. That's not ad hominem at all.
The post on question also didn't call the author an idiot, just said that the paper was idiocy. That's not ad hominem at all.