This is actually impossible without an insider’s information. Such a large funds from a cold wallet is not easy to get compromised without big help from an insider. Now that they know the group related to the hack, working collectively with other exchanges this funds may have been linked to should suffice for their losses.
The investigation was really big they even asked exch for help and some other platforms to fill the gap bybit has bought the same amount of ETH though, but on some telegram, I saw their admins trying to misguide people by saying Bybit has recovered all the funds from this hacker. While they have not, and speaking of insider information, this can't be true because they don't need one, these hackers just need a loophole in the technologies of these exchanges, liquidity providers etc. so they could break in.
Leaving small amount in exchange is better and never should one have full trust in them as your funds can never be SAFU and should have in mind that anything can happen at anytime. Bybit is a trusted exchange, we never could tell any other trusted exchange this can happen to.
You are right no one could have thought this would happen to Bybit or maybe next Binance therefore we should trust exchanges who knows the technologies they are using while I assume Bybit did not knew the nature of the cold wallet.