They don't need those 18000.

This is the biggest mistake people make.
Those coins where deposited on bitstamp by users who either wish to use them trade or to sell and get the money.

It's highly unlikely that everybody who deposited those 18000 coins will ask for them next thing tomorrow.
I assume a lot of them will just sell their coins (credited to their accounts) and demand back the usd.
So for those bitstamp can simply pay in usd.