Unless the bug being fixed was some sort of critical security failure that enabled people to game the exchange or manipulate balances. I am guessing this is more of a screw-up by amateurs than a 'take it all down until this hole is patched' situation, but there is a slim possibility that keeping it up was a bigger risk than the reputation hit of extended downtime.

No, they couldn't. It's a large trading platform. Any serious bug can lead to the theft of tens of millions of dollars worth of cryptocurrencies.