Well even if all the users deposit at once (which is highly unlikely) they simply give to every user the other users bitcoins so they should never ran out of coins... they get 2000 BTC with your example and they need to give out 2000 BTC so they don't even need to use their own supply!

About the 0 percent fee it seems not sustainable in the long run.. maybe they should change the fees or they will soon go bankrupt and will not be able to continue to mix.