Starting in the 1970's many symphony orchestras began using "blind auditions" to hire musicians. Applicants performed behind a screen which shielded them from the judge's view. Since this practice began, the number of woman in American orchestras increased from about 10% to 35%. Studies conducted since then attribute about 50% of this increase to the use of blind auditions.
If you had asked these orchestras what criteria they were using to select musicians back then, I'm sure they would have told you that they were hiring the "best qualified candidates." And yet we now know that there was a clear, although possibly unconscious, bias in their selection process.
Putting aside the merits of Ms. Pao's particular case, the notion that Silicon Valley has been hiring and promoting the "best qualified candidates" all along has no real supporting evidence. You could argue that technology firms do the best they can with the information available to them. But, that's not a valid argument to maintain the status quo.