Earlier quoted context omitted.
How is it sexism is sex is not a factor?
It is a factor.
In that case I'm missing your point. If everyone has equal opportunity to negotiate salary regardless of sex, then how does it matter whether or not salary negotiation skills should be awarded, from a gender equality perspective? Isn't that a separate issue then, not related to sexism?
Unless your point is that the sexism is at a larger cultural level that fosters the average woman to be less effective at salary negotiations. In that case I completely agree.