I don't suppose that the current quantum computers posses the capability to actually perform such a complex calculation yet. It'll only take a sufficiently powerful quantum computer to potentially use Shor's algorithm to factor the public key and derive the private keys of a cold wallet, and such quantum power hasn't been implemented yet, so using a cold wallet could still appear to be a safe way of storing Bitcoin in a post quantum world. Except I'm wrong...
You're right, the current quantum computers might not really possess the required coherence and power to perform such complex calculations that it takes to break through the cryptography of Bitcoin using the Shor's algorithm. So except a more powerful quantum computer with sufficient power is developed, using a cold wallet remains a safe storage option.