Not really. Adding up all the shares that they own, and saying that they have that much money, is completely incorrect. If any of them tried to liquidate even a small part of their shareholding the price would go swiftly to zero.
I guarantee that 100% of any of their shares would be swiftly picked up if they were sold at a 5%+ discount to the 52-week low stock price.
Markets are a real time pricing mechanism - they operate on instantaneous availability of both money and shares. Put enough shares out on the supply side, is the equivalent of flooding the demand side with money a. la. hyperinflation. In both cases, the good in question becomes effectively valueless.
btw take a look at Gates' portfolio. He's as liquid as it get's at that level.