That’s true in the absence of margin. However these were primarily margin products (“perps”) meaning the exchange has “jump to default” risk if the person on one side of a trade suddenly goes bust and liquidating the collateral isn’t enough to make the other side of the trade whole. In that case if that person immediately tried to withdraw the funds wouldn’t be there even without any kind of fraud.

Normally I believe real-life exchanges have insurance to cover this eventuality.