It could be that there is an effect which makes a certain frequency more ideal than those above of below it. For example, given a longer time to "charge" the current might be forced into a wider area.
I concur that due to lack of critical review, there is a possibility that Hulda Clark was wrong on some (or perhaps many) points. It does seem like at least a minimal pulsing frequency is required to achieve any effect at all, but that could be as low as 2-5 Hz, as she says. Where I see a possible chance of error is that there might be a window of more optimal frequencies (e.g. the 2.5 kHz range) that spreads throughout the body somewhat more evenly, while having the same effect on the parasites it encounters.
If you have doubts as to the reality of the zapper effect, obtain a zapper and use it at the first sign of a cold. Loss of congestion is a great antidote to skepticism.