The old Boyer system used to revert to full advance if the voltage dropped below around 10V. (Beware starting with a low battery voltage!)I’ve no idea if this applies to Thorspark but some electronic ignitions revert to full advance even at kickstarting speeds if battery voltage is low. This could cause your kickback?
The Thorspark system has no electronic advance, it is just an electronic trigger. It fits in the end of the existing magneto so advances using the mechanical ATD or the lever on the handlebars if it is manual advance magneto.
Tom's ignition probably just needs retarding a degree or two.