They're an exchange. Cold Storage is not their job. It isn't much of an exchange if I have to wait 8 hours to put in a sell order.
When you put in a sell order through the normal "retail" interface you're trading with Coinbase itself. Those coins can stay offline in cold storage, as they're just moving numbers around in their database, off-chain. Only withdrawals and periodic internal rebalancing operations (making up for an imbalance between buys & sells) strictly require online access to the keys, and those only affect a small part of the total.