[Miss Mercedes]

The manual says this:

Do not flat tow the vehicle with the front axle raised. Doing so may cause serious damage to the brake system which is not covered by the smart Limited Warranty.

But it's never been explained exactly what damage is supposed to happen. If the car wakes up during the tow it could freak out and lock up the brakes. But if the battery is disconnected it's not really any different than flat towing, save for the front wheels not actually moving. Looks like you just confirmed that such is the case!