Heating the fuel going into a mechanical diesel injection pump (like on older Mercedes Benz diesels) will reduce the viscosity of the fuel and very slightly reduce the power used by the injection pump (and therefore not available to the transmission to propel the vehicle). I'm not sure, but I think the laws of thermodynamics would prevent the power saved at the injection pump from exceeding the power consumed by the alternator to provide the electrical power to the heater/electromagnet.
This smells like BS.
I used to tell people that I had magnets on my fuel lines, the EN-Valve, a tornado, splitfire spark plugs, and a few other "fuel saving" devices. My fuel economy is so good, the car actually makes fuel as I drive it. I have to drain the tank periodically to prevent the fuel tank from overfilling.
OH, Ye of little faith!...............