I does seem unclear until you read the entire article. The police hadn't arrived. They were en-route, but on the other side of the interstate. Someone, Giuliani or someone else, had apparently called 911 and the police were responding. The police did witness the accident.
According to the state police, the police were attending the domestic abuse scene on I-93 at the time Goodman and Giuliani were nearing the incident coming the other way on the divided highway. Troopers saw the rear-end crash and then walked to the median to help them. Maybe we'll find out more.