Perhaps the part that you're missing is that in addition to margin accounts, Robinhood also has accounts that are able to trade up to 1k before they are funded

Perhaps the part that you're missing is that in addition to margin accounts, Robinhood also has accounts that are able to trade up to 1k before they are funded (that is to say, before the deposits clear). So Robinhood has to lay out its own money to clear those trades, as well as traditional margin trades where the customer has settled equity but the brokerage also carries risk. BTW, I don't know what that parable is but your invocation of it seems like a rather complex way of telling me that you think I'm being disingenuous.