as near as I can tell the harmonics theory is irrelevant in the Hulda Clark -style parasite zapper. she used both types, and in The Cure For All Diseases talked mainly about the pulsing kind. that is the kind she tells how to make in a shoe box. simple pulsed dc current, just like zapperplans.com or the butter knife circuit.
with beck-style ac zappers probably the precise harmonics are relevant because it targets the pathogens one at a time instead of slaughtering them wholesale. a harmonic could kill parasites not targetted by the ac zapper. you are right (imo) to question the idea of harmonics in dc zappers.