It's a tax advantage because if he had to pull the 50 million from his 80 billion, he would be taxed at 20% capital gains (based on his tax bracket). So, in essence, to get the 50 million in liquidity, he would have to hand over an additional 10 million.
But, because he has the wealth to backstop the loan (even from non-realized assets) and can get a preferred rate (also largely because of wealth), this person can get the 50 million in liquidity for 5%-10% rate, meaning the sunk in cost for him is 2.5-5 million. On top of that, if the loan is structured as a business loan, this person can also deduct the interest payments.
I'm not saying this is illegal. I'm saying that having that kind of wealth does allow these people to have much more tools at their disposal to deal with sunk in costs like taxes, which is something a person without that exorbitant amount of wealth would not have.