I have some quibbles about the ProPublica definitions -- for instance market liquidity matters when calculating public company stock wealth -- and even if you're going to borrow against it, there are additional costs and pledges that must be made that significantly reduce the available capital.

The propublica number was like 4.5% or so if I recall, and does not count the taxes paid by the companies these people owned, nor does it imagine the financial benefits to say California teachers or firemen who co-own the companies through pension funds, nor does it reduce for effective wealth, nor does it reduce for unutilized wealth, e.g. if the stock price goes up and you don't sell or borrow against it, have you received benefit that makes sense to tax?

But if you net all those out and told me the effective rate was 12-15% on utilized capital, I wouldn't be surprised. I would be really surprised if it was $0 though.