Did they ever stop to think that maybe people who are talented and interested in starting their own tech startup business might seek out these schools and positions, disproportionately to other options?
You can't point at facts like these and claim they are proof for a lack of diversity. It might turn out that, given the chance to evaluate and interview every possible candidate in the world, that the top 1% of talent might be 80% drawn from those backgrounds, irrespective of all other traits.