It's true. When decelerating in gear or descending a hill in gear with your foot off the accelerator the car's fuel supply is completely cut off and, effectively, the wheels turn the engine so it uses no fuel at all. You can see this on the car’s ‘instant consumption’ read-out. When coasting out of gear the engine uses a little bit of fuel to idle and stay running. The difference in consumption between in gear descents and coasting is actually tiny, but it is far safer to descend a hill in gear than to coast out of gear.