I guess the same thing that puts the energy imbalance required for lightning to happen, is being used here.

I also think based on the small scale this study was done, there would be a very small amount of electricity generated - enough to maybe power a sensor. Otherwise there would be lightning. We already have 'energy harvesting' buttons and sensors which generate electricity from what would otherwise be wasted (I have a battery free doorbell which gets enough electricity to transmit a radio signal - from the act of pressing the button). This might start out as another way to get electricity to these kinds of devices.