I think GP meant that "aggressive screening by gender" is another way of saying "discrimination". In the US, I think that's technically true -- you can end up with a very different pool of hires than applicants, but you're vulnerable to lawsuits forcing you to defend the merit of the ones who were hired.
I'd be much more interested in learning what Gusto changed (if anything) and how it changed their applicant pool.
Were 50% of the applicants female? Was there a significant quality difference between genders or across time?