I don't know how accurate their readings are, but it's (like you say) not the frequency that is unique, however the exact shape and intervals of the PQRST point

I don't know how accurate their readings are, but it's (like you say) not the frequency that is unique, however the exact shape and intervals of the PQRST points within a single ECG period is. See also: https://en.wikipedia.org/wiki/Electrocardiography https://en.wikipedia.org/wiki/Electrocardiography