The devil is in the details.

400 watts of resistance heating is 400 watts of resistance heating, no matter how you advertise it. It is no more, or no less, efficient than any other 400 watt resistance heater.

Heat pumps can be more efficient, though.

Is 400 watts enough heat for a 10x10 room? It depends on how hot you want the room, how cold it is outside, how many times the air in the room is exchanged with outside air and how good the wall and ceiling insulation is...

If it sounds too good to be true, it is too good to be true.