Not a big deal, they have so many coins sitting in the cold storage collecting dust. Their reserve ratio is much higher than any banks in the world, which typically have only 20% of customer funds at hand while still serve the customer for decades
Banks do not work that way. Banks make loans, which are assets to a bank. If you own a loan, you can sell it, which banks do. Loans plus cash plus capital must exceed deposits. Trouble can appear if a broad class of loans lose value, which is what happened in 2008.
Bitcoin exchanges have no secured loans as assets. They are not banks. They are not lending institutions. They have to have 100% of their deposits or they are insolvent.
What you said is academically true. But behind the scene, banks and exchanges work exactly the same way: Customers only play with numbers in their database until they withdraw real money. That's the reason M2 and M3 can be magnitudes higher than monetary base since they are only some numbers in bank's database
Bitcoin is hard assets, you could consider those hacked coins as a long term loan to hackers. Even so, bitstamp still have 80% of customer funds at hand. But in a bank, they never keep more than 40% of the funds at hand and their long term loan to business can not be easily liquidated. When a wave of customer withdraw hit, banks reserve will be drained quickly, and they have to borrow from other banks at the cost of interbank short term interest (LIBOR), but if majority of banks are facing such withdraw pressure, they will turn to FED asking for new base money injection
In bitcoin world there is no lender of last resort, so if everyone withdraw from bitstamp, they will have a bank run, but I think with more than 80% reserve ratio they are much safer than most of the banks
I remember that after 2011 MTGOX hack, they proved that they had 400K coins in a cold storage, and finally they lost 800K coins, so it is very likely that they were operating in a 50% reserve basis for almost 3 years
