if they did not keep their money in ordinary wallets, and wallets with multi-signature, it is possible they are with such a problem and did not face
That's not practical for the funds users want to withdraw. There has to be some that's instantly available. For Coinbase it's usually 2% of the funds they control. The strange thing about this hack is that it took so long for them to notice which means that maybe they weren't all for instant access.