Not necessarily.... When decelerating, the engine RPMs are higher and more air is being pulled through the engine. In order to keep the fuel/air ratio in the range that prevents excess NOx emissions, the computer has to add extra fuel. With carburated engines, the extra vacuum causes additional fuel to be pulled through the idle circuit.
In my carbureted Mazda, a solenoid valve shut off the idle circuit whenever the car was coasting in gear with the accelerator released and engine speed above idle. That's with 1980 technology.
When coasting at lowish speeds, the Prius effectively gives a choice between engine-off coasting in neutral, and engine-off coasting with light (if you stay off the brake pedal) regenerative braking. At higher speeds, the choice is between fuel cut with engine being rotated at idle speed by the wheels, or same plus regenerative braking. Using regenerative braking is wasteful, except when you need to slow down.