Coinbase definitely doesn't keep all their funds in a hot wallet. They keep enough readily accessible to cover the projected volume of withdrawals; the rest can

Coinbase definitely doesn't keep all their funds in a hot wallet. They keep enough readily accessible to cover the projected volume of withdrawals; the rest can be kept offline. They even specifically offer a "vault" service for managed cold storage. 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.