Earlier quoted context omitted.
Cars in motion → stationary are different from completely stationary objects, which are ignored due to high false positives.
If there is a stationary car and I turn on autopilot as I approach my car doesn’t ram into it. If I turn autopilot on when I’m stationary and the car in front of me is stationary it does not drive into the other car. If autopilot is on and a car is stopped at a red light in front of me my car does not slam into that car. I do this on a daily basis. Do you?
The radar returns on stationary objects are discarded AFAIK because of poor radar resolution leading to inability to precisely locate objects both horizontally and vertically, leading to roadside and overhead signs being mistaken for cars.
Common auto radars have limited vertical discrimination, and this is partly intrinsic due to radar wavelengths.