Because they are saying JOKs DNA was found on the vehicle...so the vehicle MUST have hit him. Unless one of the RSCs dripped on the vehicle.
We know JoK was driven by KR that night, he would have regularly been in the car, he was on the passenger side, finding is his DNA on the car IMHO, is expected and not a sign of guilt.
(Not sure about this, but I remember hearing previously (ie before this trial) that cops have a habit of touching the rear of cars.) Why Do Policemen Touch Your Tail Light When They Pull You Over? if it is habit of cops maybe JOK still followed that old habit)