I'd sure rather have an Elon Musk or a Mara Aspinall in YC than a randomly-selected person who already believes YC is the best (or only) way to do a startup.
There are two parts to this -- making YC as useful as possible to the best founders, and making sure the best founders know that and apply.
I think you're missing the point that there could be reasons that females (or any demographic other than white American males) do not apply to YC that are not correlated with building great companies.
For example it is plausible that woman feel alienated by the tech startup scene and avoid applying to accelerators. White males born might not have that feeling. So effectively their "bar" is lower.
If YC is trying to address that it seems like a smart business move on their part. They are reducing irrationality in the system.
To me, regardless of gender, YC would only want those that were proactive and dedicated to join a YC batch. It seems pointless to encourage specific demographics that would not apply, to do so.