Money is fungible but how its accounted for isn’t. I’m not an expert in the regs around margin accounts but have worked in other spaces where my company was holding money for others.

It is usually strictly regulated. In some jurisdictions you must hold it in specific accounts that have particular attributes and you must have dollar for dollar parity between the account and the money you collect. In almost every situation where you are holding money for someone else you aren’t allowed to use it to pay your own debts. And in this case the collateral obligation is on Robinhood not on their account holders.