Can someone explain how you can take out a ~$150m flash loan? (Did he post $300m collateral?) Did he only need 3 ETH for that or were the ETH only used for the transaction fees?
With a flash loan, the funds must be returned by the end of the transaction, or the transaction fails. This makes the completion of the transaction the collateral, as if it fails at any point, all transactions (including the loan) get reverted.