Coming in here late, but here's what they're saying:

Ideas of gender arose from sex differences in the past. There COULD be a natural difference in how those sexes act, but because we live in a world that is already constantly defining and redefining gender roles from a cultural perspective, you are very unlikely to find it. It's like talking about "true human nature". You will never find what that means outside of the context of the world today because every human alive is influenced by countless generations of culture that have shaped how they see the very idea of "human being".

They aren't saying sex plays absolutely no part in how one feels they should express their gender, they are saying it is a pointless question that can't be answered at the end of the day.