I believe Coinbase holds the majority of their BTC in offline wallets distributed in multiple bank vault around the world. They are probably one of the groups

I believe Coinbase holds the majority of their BTC in offline wallets distributed in multiple bank vault around the world. They are probably one of the groups with the best BTC storage security (while maintaining usability). So yes, if they WERE to be robbed, then that would basically mean that no one is able to properly secure BTC.