I think it's fairly common for people to claim that a preference for thin women is genetic when it suits their purposes. I don't think many scientists would actually make this claim, but I think a lot of amateur ev-psych types would.

The fact is that thin-ness as currently defined by Western society is not adaptive; in particular, I would be surprised if it is not the case that a very large proportion of fashion models are amenorrheic. For women, maximum fertility probably occurs at a higher body fat percentage than is generally considered "ideal" by American standards. Similarly, maximum fertility occurs in the mid to late teens, but I've definitely seen people argue that the male attraction to younger women causes them to prefer women in their early twenties. This may be true versus, say, women in their thirties, but if reproductive fitness were the whole story, men would really prefer fairly young teenagers, which I don't believe to be the case.

Since Wrongbot's whole point is basically, "There is a serious problem with the whole just-so view of evolutionary biology, which has been whittled down and fudged to justify our socially constructed view of what a 'proper sexual relationship is," that point is well-supported by this claim. The fact that biological preferences clearly exist - like clear skin and a .7 WHR - is only evidence in favor of Wrongbot's main point.