They just use a hall effect sensor in it.
I don’t know that it would be all that practical for telemetry to the ground station because it’s not going to work on multi-rotors with numerous ESC’s. For helicopter or plane it would. But with more and more radios supporting telemetry these days, it makes more sense to feed the data to the RC radio, and not the ArduPilot ground station (to my way of thinking).
It is really the only logical method to use for high-power, high-voltage helicopters for power system telemetry data. The AttoPilot modules can only solder 12ga wire and I got 10ga coming out of the packs on my 12S heli’s.