To me, the choice of words makes a difference here.
I would use injury to refer to a laceration or a contusion, anything that happens rather suddenly.
IllThe word "ill" feels like a better fit when talking about a condition of pain and suffering that lasts. The speaker wanted to express that the man is having severe health problems as a consequence of the accident. "Injury" would be a good word to describe the trauma at the time of the accident. "ill" however feels like the proper word to describe the condition he is in after the accident.
The speaker could've said:
A man is seriously ill after having been injured by a police car.
He got hit by a car -> he was injured -> he lost a lot of blood -> now he's in the hospital, seriously ill.
Even though he's obviously still injured as well, the state he now is in is better described as being ill. His condition has simply gotten worse over time, either as a direct consequence of his injury or for reasons not (e.g. an infection) or only partially (blood loss following the injury) related to the actual accident. To convey that this development occurred and isn't necessarily directly related to his injury during the accident, the speaker chose to say that he's seriously ill.
Maybe that helps clarify it for you.
You stated that you wouldn't say "ill of an injury". I think what the speaker meant in this case was "ill due to the consequences of an injury".