It does not say low-power. It does say efficient, but calling an electric heater efficient is kinda like saying that a boat is really fast on land.
Efficiency, in the sense that it is normally used, does not apply to heaters because what they are used for is the opposite of how efficiency is normally defined. That is, an efficient device is one that converts as little of the power going through it into heat as possible. In the case of any resistive heater, the efficiency is (by design) essentially zero in the traditional sense, and even if you make up a special sense for heaters in which efficiency = heat produced / power input, the efficiency of the heating element itself will always be 100%.
Now, some heaters may be able to use less power to produce the same heat, but they would do so by having better insulation (less heat loss) or by having less thermal mass to heat, or some other strategy to keep the heat where you want it without bleeding it off to some colder adjacent space by conduction or convection. The heat produced is still governed by Watt’s law.