The exchange should know that hackers can get to their cryptocurrencies without the exchange getting online or not.
The exchange should be aware of this, but it's most unlikely to happen. Because of the hacks which have happened, they always access the exchange system and authorised withdrawal, not with a wallet key but with online approval, which is only possible on their hot wallet. Since the exchange is moving their funds to a cold wallet it will require the hacker to gave access to the wallet keys, as exchanges with large sum won't be using a wallet protected by only one key excluding passphrase, & multisig.
Maybe it's a hoax, maybe it's real and that's the protection that they've done for their customers. But for how long they'll think that it's safe to get back on operations?
They did not give the actual time. It would be, my guess is, until they have carefully diagnosed their system to make sure there is no threat and maybe change their wallet access. Until then their customers will have to suffer.