I had foolishly thought that the proposers of the electric comet idea knew a bit about electrical discharge machining
. It seems that they do not.
: EDM removes material from the surface of comets and creates water from the material somehow.
If there is no dielectric material between the electrodes then there are no sparks and there is no machining.
In the EDM idea the workpiece-electrode is the surface of the comet. I do not know what the tool-electrode is (the coma?, the tails? the solar wind? maybe the Sun!).
There is no dielectric material above the surface of a comet - just lots of highly conductive plasma in the coma, solar wind and tail.