The battery itself is covered in a non-conductive PVC vinyl which is why the arcs hitting it don’t impact it at all. The CD substrate is about the thickness of a human hair, and wasn’t made to be nonconductive so it offers minimal protection to the foil underneath (which is put in the direct path of the two terminals).

Holy crap that article splash image is going to haunt my dreams tonight...

Electricity follows the fastest path to ground. Neither the battery nor the motor are included as a part of the circuit, so the electricity ignores them essentially. Now, insert your nice moist hu-mon fingers in that path and you’re screwed, but the nice plastic spinner those were sitting on had no means of inserting…