Do you agree that negotiation skills don't matter for most jobs? And do you agree that people should be compensated based on the value they bring to a job?

Because sexism or no, I think it's a tremendous problem when people get ahead for completely tangential reasons to their job performance and skills.

I agree it's a problem. But it's women's problem. If they want to get ahead - start negotiating. It's not incumbent upon society to bend over backwards to facilitate every subgroup's predilections. Salary negotiation is a necessary part of the job market. It performs a useful and important function - price discovery. It is not sexist if women are maladapted to it.

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.