Not so. Keeping a constant distance from the car ahead means both cars are moving at the same speed. When a jerk cuts in, after a moment all 3 cars will be moving at the same speed.
We are saying the same thing. When a jerk cuts in, drivers readjust their speed to maintain desired following distance. Net effect, slower speed for all but the lead car
If you personally start with that slower speed to begin with (AKA much longer following distance), you don't have to worry about adjusting down
