You're right! They can't just walk into a bank and withdraw 10 million!

Instead, they take out a loan for 10 million against a collateral of 20 million in stocks, expense the interest, claim 0 income and therefore no income tax, and have that transferred to their bank account. Then they withdraw 10 million, while their stocks continue to gain (or lose) value, and at some point they have to take some portion of their dividends as income rather than rolling them into more stocks or, God forbid, sell some of those stocks to pay back the loan.

In a technical sense, no, that money isn't just sitting in a bank account, but there's a weird effect. If you only have a small amount of stocks, say a few million worth, you can sell them off in a relatively short period without harming the stock because it's just a blip at most in normal volatility. If you have a lot, say billions, you can sell enough to do whatever the average person would ever want to do with only slightly more volatility than is some nobody did it, and much of that is due to the visibility of a billionaire doing that. If that billionaire needs more than a few million, he can just follow the process outlined above with no impact at all to the stock market and a number of benefits to himself. So in a practical sense, if you only have a little stocks, say millions, accessing those millions may take days or weeks, but if you have billions, you can access it only slightly slower than I can withdraw $100.

This money is still in the economy and helping to economy to function overall, but the majority of that benefit is only experienced by one person, the billionaire. This is effectively them hoarding those billions, not because they're sitting in an account but because a single person is experiencing the majority of that benefit while the rest is spread across thousands or perhaps millions of people.