In this case, the radar system itself is moving... including moving relative to stationary objects. So I don't see how what you say makes sense.
Not saying you are wrong. Just saying I don't follow your explanation.
The real issue is false positives from things like soda cans on the road, signs next to the road, etc. Can't have the car randomly braking all the time for such false positives. As a result, they just filter out stationary (relative to the ground) objects, and focus on moving objects (which are assumed to be vehicles) together with lane markings. This is why that one Tesla ran right into a parked fire truck.
Interestingly, I've discovered one useful trick with my purely camera-based car (Subary equipped with Eyesight): if there is a stationary or almost stationary vehicle up ahead that it wasn't previously following, it won't detect it and will consider it a false positive (as it should, so it doesn't brake for things like adjacent parked cars), but if I tap the brake to disengage adaptive cruise control and then turn the adaptive cruise control back on, it will lock on to the stopped car up ahead.