At that time, my account got about 5 million USD accounted from some small BTC sale. Later, the MtGox ticker showed price at 14.10 USD per BTC. So, theoretically, I could buy plenty of BTC ... If I wanted.
Perhaps, MtGox had same safety blockage applied. But if not, anyone could rip the BTC out of it.
They picked up the issue as soon as it happened, you would have had to do the trading to BTC and withdrawing it within a few seconds at best.
Remember, as they patched and looked at system the issue occurred thus they could shutdown trading immediately.