In theory, that sounds about right. In practice, each smallish team will be culturally independent, and there won't be enough data to show off how equally your company is treating employees (or it will treat them all equally bad, but that's a different discussion). Let's remember that majority of the companies are small, and not everyone will go out of their way to publicly commend internal policies.

Just imagine a 20 person company with 5 women who are being fairly treated: how do they get better gender balance? Even if these five persons were outspoken, how many people could they reach? Not to mention that a true character of a company is only revealed in a crisis (harrasment, known unequal pay, role progression...). This company will never get the breadth of candidates Facebook might to be able to rely on statistics. Without actively pursuing that as a goal, balance never happens.

Positive gender discrimination is a societal tool, a tool to fix unfair social treatment of genders at an early age ("here are pink dolls for you dear, your brother can go play games on the computer" just as a blunt example, but it's a lot more subtle in general).