Your link 404's. I'd imagine that the heater would be designed so that when someone forgot about it being in a cup of water, the water could boil dry and the heater would be exposed to air. Something would have to reduce the power consumption or shut the power off completely (probably a simple thermostat). Thus, my guess is that the thing probably won't work in air.
But there are plenty of small heaters available, so it shouldn't be hard to find something appropriate. You could even use an incandescent light bulb or two, as they're pretty efficient at turning electrical energy into heat.