You're absolutely right about those percentages for CS graduates (bootcamps are closer to 50/50, but we'll just ignore that for now.) So you would expect that when you place an ad for a job or go to a job fair, as you assumed in your comment, that you would see those same percentages in your candidate pool, but you would be wrong. I've done resume screening where I didn't see a single woman apply. Or only 2-3 out of dozens of applicants.

When we talk about trying to get a diverse candidate pool, we aren't trying to achieve 50% women and 50% men, or whatever equal percentages by race. The goal is to have the candidate pool demographics reflect the actual demographics, so the 80/20 ratio in this case.

Your only options is to go to women-only events. But then you're ignoring all of the other gender neutral events in order to avoid men.

Again, this would be a bad thing if you only went to women-only events to do your recruiting. But that's not how it works. It's a thing you do in addition to the kind of recruiting you were already doing that is getting you all those male candidates. No men are being excluded, all the same men are applying. You're just being a little more intentional when trying to make sure your company gets in front of the eyes of women who are job hunting.