He mentions that 80+% of his assets are now in startups. Having that much money tied up in assets that have no scheduled exit events seems like a pretty strong argument by itself.
Early investments in Uber are up something like 10,000x. That's such a large gain that only a super wealthy person could have 20% of that in liquid assets. He probably has much more than 80% tied up in unrealized paper gains. And those gains are dominated by Uber.