Very good question.
Ford uses an accelerometer in addition to the brake pedal to determine when to activate the brake lights. I have had my wife drive behind me and talking with me on the phone when I was driving the Mustang Mach E in 1PD. She reported the brake lights coming on exactly when I thought they should. It is based on the rate of slowing by the car, so a very gentle deceleration/regeneration doesn't light the brakes but backing off the accelerator enough to feel a significant slowing does. You definitely see brake lights any time you let off the accelerator completely.
