I hate to break it to you, but that's not true.
Yes, the company tells you that's how it works. But I was also a Google engineer, I did hundreds of interviews during a time when it was way less into diversity than it is today. And absolutely Google bent its policies to try and hire more women. Constantly.
Some tricks I saw used:
1. Assigning the best/most reliable interviewers to women. Recruiters confirmed to me that they did this.
2. Allowing women who failed the phone screens to proceed to on-sites anyway.
3. Organising recruiting events which banned men, e.g. there was since early years Google CodeJam, a coding competition that was pretty openly used as a way to find candidates. But the men crushed the women, repeatedly. I think there was a female finalist once in 13 years. So they set up a second CodeJam with large cash prizes, but banned men from it.
In recent times there was the recruiter who quit and claims he was told to not hire non-diverse candidates. I believe that. I was told about recruitment's diversity goals by recruiters themselves.
But still, there was a lot of resistance to bending the hiring pipeline in more extreme ways. That culture carried over to Facebook. I've been told that at Facebook recruiters were incentivised with cash to hire more women, but they gave up pretty fast because stuffing the pipeline with weaker candidates didn't work: they got flushed out by the hiring committees.
By the way you're blind to a much more important trick Google uses. Eng hiring was, during my time, at least somewhat well defended against diversity abuse. But firing was totally controlled by HR. And it is an open secret that at Google it's nearly impossible for women to be fired, regardless of how low their performance is. Instead they get moved around between teams. Men, on the other hand, can be and are let go for merely not exceeding expectations. Obviously that's one way to get less male workforce, if you're patient!