so how the heck can he still get margin / leverage after being banned from trading?

dude borrow money to gamble on stock market, stock market go down, banks liquidate his holdings (margin call) to avoid losing money, market sold off even more.

moral of the story, don't borrow other people's money to gamble.

margin is the loan, when you hear people buying on margin that means they are using the broker's/bank's money to buy stocks.

margin call is the brokerage calling you to top up your margin due to your account not meeting margin requirement, if cannot they force sell your holdings and you are stuck owing the brokerage/bank.

margin is a way to leverage your buying power, if you confident that market will go up or down and use margin properly you can 10x 20x your returns. it's a double edge sword, you can also lose 10x 20x if you bodo like this bill hwang dude don't de-leverage.