I took a gender studies course (I know) in college and did not interpret it that way. It's less that they're saying, "society completely invented gender" and more "gender exists only on a social level, therefore it is mutable." In other words, instinct is biological, but instinct can be ignored/overwritten. Woman have a tendency to be one way, and maybe genetics are responsible to an extent, but "women are always x" is not incontrovertible truth. To me, I've always seen postmodernism as being about how there ARE NO incontrovertible truths, even if we want to believe people fit into these neat little categories, they don't, because THOSE CATEGORIES are constructs. The traits by which we categorize may be biological, but the categories themselves are not.
The underlying idea is simple but powerful. If we are trying to explain some phenomenon, X, then we need to identify variations in the likelihood of X or the rate of X, and look for potential causes that (1) vary across the relevant circumstances in a way that could explain X and (2) that we can connect to the outcomes for X in some way. For example, with the gender distinctive clothing question, some ways to better specify the question and look at it through comparisons are: