When the engine is connected to the wheels through the gearbox there is no fuel consumed on overrun, as you correctly say. The kinetic energy of the car is, however, reduced by compressing air in the cylinders of the engine and turning it into heat. Think how difficult it is to turn an engine over by hand.
When the engine is disconnected from the wheels it is true that a small amount of fuel is needed to keep the engine idling. However the power (energy times time) taken from the fuel is considerably less than the power (kinetic energy times time) taken from the car if it were connected to the wheels. This is simply provable by the increased speed of the engine which runs above idle when the engine is connected.
When I discovered Eco mode last week I was surprised and impressed by how far the car travels in E without the engine connected.