They are picking up the electromagnetic signal of arcing and not fault voltage or the potential difference of the pole to the earth. To measure or detect voltages at least two reference points are needed. There can not be a voltage difference if you do not have a difference. One may argue that they are detecting an electric field which does not rely on a current. That is possible but a universal axis of multiple detectors are needed. The electric field is polarized. I know of no electric field detectors that would work at the distances shown and do it while moving. Magnetic fields or electromagnetic fields can be detected and that is what they are detecting from something serving as an antenna but most probably from arcing.