Moreover the OP missed (as did most of the readers here) a too-subtle point made in the summary: it's not about who's better at negotiating, it's about the fact that culturally we (usually) are comfortable about men being pushy about their salary, while women tend to be treated negatively if they do the same thing. It's likely not a conscious decision on the part of those they try to negotiate with, more an unconscious reaction to a difference in expectations, but ill intentioned or otherwise it does actually happen.
I know women I work with who are considered "difficult" by all the (male) colleagues around me, simply because they do actually try to get ahead. For the example I'm thinking of, there's literally nothing she does that isn't done by far less qualified male colleagues who end up in more senior positions. But nobody wants to work with her, because she's "pushy".
We're rewarding people of one gender when they negotiate a salary. We're punishing people of the other when they negotiate a salary. Surely even Slashdot's current infestation of MRAs must see the problem with that.