Sure, it'll never be a significant source of energy [if we plastered literally the entire earth's surface with them at maximum theoretical efficiency, we'd provide 1% of our electricity consumption].
But the sheer concept of "you know what, lets generate energy from THE VOID OF SPACE, the ABSENCE OF HEAT" is deeply entertaining.
But then...go outside a night and point it first at the ground, then at any clouds, and then right up into darkness. You'll see a big drop in temperature, which is the absence of heat energy in the atmosphere itself...all the way out into space.
I believe that the sensor has some type of angle to what it can detect, so you're getting the average reading across a large cone of air/space at that point. Also, the radiated heat along the depth of the cone would be "stacked", so that you are viewing a 'cumulative' reading from the entire depths of that cone, like an integral going into space.
In practical terms it probably isn't efficient enough to be worth the weight, but the idea is neat nonetheless.