Idle fuel flow increases with engine rpm. If it didn’t, the engine would have to be restarted after bringing the car to a stop with the transmission in gear. If you’re going to save fuel by coasting, do it right and disengage the transmission.
Incidentally, if you’re using the engine to save the brakes, this is no longer considered beneficial. Back in the forties and fifties, competition sports car drivers used the engine for this purpose, but, with the disk brakes and friction materials used today, this is no longer considered necessary. Oh, they’ll still take advantage of engine braking when convenient, but you no longer hear them coming down through every gear as they once did.