I think some of those variables can be tweaked to encourage more e.g. women to apply. I know that women (and probably other minorities) are less likely to apply for a job they don't feel qualified for than men. I've noticed some JD's stating to apply anyway -- I always appreciate that gesture.
What I mean is that it can be a copout that hiring managers use and is hard to argue with. Almost everyone will agree that hiring the best for the job is the desirable outcome. It just takes for granted that all qualified applicants were even considered.
True, which is why I made the statement narrow -- we hire the best applicants. I make no claim that the applicants are the best devs available in the industry overall. "Best", after all, is context-dependent. A top notch engineer may not be the best for a given position due to other factors.
The point I was trying to make is that we hire based on ability, not based on unrelated factors such as trying to hit arbitrary diversity goals. For these companies, the diversity comes naturally as a result of not caring what the race/gender/age of the applicant is. I have no idea, and make no claim about, whether or not the experience at these companies is an exception or a norm. It's just what I've personally seen.