Correct me if I'm mistaking, but the ESC runs on a PWM (pulse width modulation) signal from the receiver, I know the servos do (ever plug an ESC into the wrong channel by accident? It still works correctly)
PWM works by turning on/off extremely quickly (it only feels like partial throttle)
I know that in servos, the interval is 20 milliseconds and an "on" period of 1-2ms controls what angle it turns to.
For an ESC, it has some as equation involved to make the 1-2ms signal stretch from 100% forward to 100% backwards. If the ESC knows the resistance of the motor (witch I believe is closely related to the kv rating) then it can take the voltage it reads and resistance of the motor and limit the PWM at a certain percentage.
Or at least that is my guess, but it does seem like a crude way of doing it. If I knew of a current sensor that worked in DC and didn't take much space, then I'd say it takes a reading from that.