First, I don't think they claim a 50/50 split. I guess there's still room for improvement even before hitting the applicant pool limitation (I have seen it, too, BTW, when we hired; the number of women applying is higher than their representation in startups).

Second, it's also very possible that there is a local, fast, feedback loop. I think women would be more attracted to companies that already employ a higher number of women, and are known for a hospitable working environment, so it's likely those companies will receive a higher share of women applicants.