I interpreted the quotation marks around "social construct" to symbolize sarcasm, not an actual quotation.
You might be right. I am trying (and failing) to remember what he said in the Madness of Crowds on this subject. There were definitely a lot of parts that I strongly disagreed with, even though I found it a really engaging read.
Personally, I'm actually a bit agnostic about gender being a purely social construct. It might be that there are some behaviour differences between the sexes although I think it's frankly impossible to tell in our society what, if anything, is driven by biology rather than pink/blue drivel and pressure on women to behave in a certain way.
But what annoys me is that even if gender were socially constructed, it does not make it legitimate to opt in to the gender trappings associated with the opposite sex. To use an analogy, there are a lot of socially constructed behaviours around race (Ie a shared ethnic culture including music, food, language/slang etc), but that doesn't mean that as a white person I can opt into them and claim to be of that race. "It's a social construct" shouldn't provide an entitlement to declare yourself to be something other than what you are.