The $10M figure is incorrect because donating a $20M painting doesn't save you $20M in taxes, it only allows you to deduct $20M from your income, and even that amount is capped. The actual amount of taxes you save is roughly $20M * your marginal rate. There's also the $10M you spent to acquire the painting, which eats into the savings.

That said the general premise is valid. The loophole comes from the fact that when you donate something, you don't have to pay capital gains on it, yet you can deduct the full value from your returns[1].

[1] "If you donate long-term appreciated assets like bonds, stocks or real estate to charity, you generally don’t have to pay capital gains, and you can take an income tax deduction for the full fair-market value." https://www.fidelitycharitable.org/guidance/charitable-tax-s...