I don't know if I'm right or not but I heard somewhere that Coinbase's most Bitcoin's lie in their offline cold wallets that no one can hack unless they get to it physically get access to it. And, that could be true for this statement as well. If they have stored most of their Bitcoin assets to offline wallets then that's a good security measure and somewhat hack proof.

Let's say if 10% or $10 billion of their Bitcoin assets are available at their hot wallet and it somehow gets hacked then that could cause huge problems for them and also for the market in general. That could have very hard impact on the value of Bitcoin in general as most of the weak hands will keep dumping their Bitcoin at market.