If they truly have cold storage of their btc - yes, this system does improve security, as their cold-storage bitcoins aren't subject to electronic hacking, just physical intrusion. And, the use of encryption and HSMs can mitigate against even a physical intrusion as well.

Sometime in the next couple years, someone is going to come up with a relatively fool proof vault storage for bitcoins, we just haven't see it yet. (And Coinbase isn't sharing enough details yet for us to know whether they've put one together)