This assumes white engineers only knows other white engineers etc.

This is very much not how the Silicon Valley I've been in for decades works!

Of course, you could have diverse acquaintances and friends, but most people are more inclined to be friends with people who share the same race and gender because they often share the same experiences. This puts a black female at a great disadvantage if her friends are mostly other black females because so few of them work in tech.

I would only refer people I've worked with. And they are as diverse as the industry.

This approach does discriminate against people who are looking for their first job. I think that's both unavoidable and fine.

This gap is covered mostly by internships.

that is not bias, but rather a lack of diversity

It's still a bias - one that's exacerbated by a lack of diversity