Here's a patent for a different type, they make the handle conductive so they can use the body capacitance as a decent ground relative to the capacitance of their capacitive divider (ie. they make the divider << 100 pf).
https://patents.google.com/patent/US9829515B1/enThough that still doesn't explain how the hotline one works. AFAICS the capacitance to ground will be significantly smaller and less predictable, making everything harder.
PS. I guess I'm making it harder than it needs to be, I guess it's just a resistive divider and a peak detector, treating the ground plane stray capacitance as a very temporary earth. I was thinking all the parasitic capacitances of the components would mess it up, but I'm probably overthinking it.
PPS. works well enough in simulation as long as capacitance to ground is more than a couple pf and the capacitance across the top resistor is small enough.