I once said that you need to store Bitcoin, namely, you knew about your cold wallets, but you used other exchanges.
You can't expect everything to be stored in cold wallets. They stole a very small number of coins and as long as the loss is small it can be reimbursed and won't affect the business that much. If you have 1000 Bitcoin on your platform it's natural that up to 10% will be in hot wallets but some businesses like that Korean exchange that was hacked had all of their money in hot wallets.