I'm not doubting Timothy's experience, but if he's right, the point would have to be that VCs bring-in-Biff more often with black founders than they do white ones. Which seems entirely plausible, but that's not the argument I think I'm reading here.
Even if their selection process for Biff was completely gender / race blind, as long as the primary qualification for Biff is "has previously been CEO" then the next generation of companies will continue to inherit the previous generation of companies' demographics.