The poster you’re replying to is talking about something else. There’s a point where the terms you can get for loans using your stock portfolio as collateral are so good that you can count on your stock value growing faster than interest payments on the loan, enabling you to take out loans that amount to free money and live off of them (or use them on more investments that grow faster allowing you to take larger loans, etc).

Banks don’t mind because they reliably get their interest payments, can count on settling the account when the person dies, and of course there’s the social capital of being the institution that ultra wealthy people bank with. For an ultra-rich person it’s how they can have the liquidity to live an ultra-rich lifestyle even if all of their wealth is tied up in the market.