Radar is not good for detecting stationary objects. Of course you get a nice reflection back from a stationary car but you get a similarly nice reflection from an overhead trafic light or a manhole cover or a dropped nail. Because of this every automotive radar ever fielded gates out the stationary objects. If you wouldn’t you would get a crazy amount of false positives.
They can do this because the radar measures the relative speed of objects via dopler shift, and you know the speed of your own vehicle. Anything which has the same speed as you have but goes in the other direction is most likely stationary. (Or moving perpendicular to you. The velocity difference is vectorial, but dopler can only observe the component along the observation vector, and civilian radars have terrible angular resolution.)
In short: nobody ever used radar to stop cars from hitting stationary objects. This is not a Tesla specific thing.
They can do this because the radar measures the relative speed of objects via dopler shift, and you know the speed of your own vehicle. Anything which has the same speed as you have but goes in the other direction is most likely stationary. (Or moving perpendicular to you. The velocity difference is vectorial, but dopler can only observe the component along the observation vector, and civilian radars have terrible angular resolution.)
In short: nobody ever used radar to stop cars from hitting stationary objects. This is not a Tesla specific thing.