Even if only 5% of his NW is in cash, that'd be $50billion dollars in actual liquid cash. Even if it's 1% or less, he almost certainly has over $1bn in cash or practically liquid cash. That's already a dragon's hoard.
Which is why the term "cash," to those in finance or familiar with its lingo, is understood to actually mean "short term treasuries" and other things you would find backing a money market account, unless the context very specifically calls for distinguishing those two things.
There are no Scrooge McDuck cash vaults.