Very few engineers in Silicon Valley interact with black engineers. That's a problem. You can't have positive experiences to remove your bias if you have zero experiences.
There is little diversity to be gained from black engineers. Black engineers end up in the category of another nerdy American.
Race is not a good metric for diversity.
Diversity would be Arabians, African immigrants, Thai, etc.
Who is to say they have any? You're implying they're somehow racist because they haven't met a black engineer? Just because of their skin color you're gonna judge 'em? Well sounds like you're the racist.