Eric is not suggesting that the applicant pool is diverse and the selection process biased. Instead, I think he's suggesting that different selection processes can result in different applicant pools:

"Imagine that you were a professional musician thinking about which orchestra to audition for. You have a choice between an all-male orchestra that conducts interviews out in the open, and a mixed-gender orchestra that conducts auditions behind a screen. Which would you choose to apply to? Wouldn’t your answer be different if you were a man or a woman?

I think thought experiments like this are helpful for suggesting an alternate hypothesis to the pipeline problem: that there are qualified minority applicants who are choosing – rationally – to invest their time and energy elsewhere. I am not aware of any scientific study that proves this hypothesis is correct. But I have seen enough existence proofs to believe it is likely."