You're right of course, but these days engine breaking is most commonly used by tow vehicles, whether that's a family car or a big hauler. Either way when towing, these are generally required to have trailer brake controllers that are activated by deceleration so that the trailer brake lights do come on.

When we cross the Cascades or Rockies in our Land Rover Discovery towing an Airstream, we have to shift and engine brake on the downslope for miles (to avoid overheating the breaks) and it's noticeable that the trailer brake lights go red, while the Land Rover doesn't unless I tap the brake pedal.