There was an excellent documentary done on this. The police chief of the town said that he believed that Kennedy was not in the car at the time of the accident. The retired police chief said that an officer patroling the area where the accident happened called in on his police radio reporting that there was a large park parked in the dark. When he drove by the car took off.
The police chief believed that after the officer drove by, Kennedy got out of the car and Mary Jo drove the car. This makes sense since Kennedy was probably drunk and would not have wanted to be caught driving drunk.
If I recall correctly Kennedy said He WAS in the car so no it doesn’t make sense if that is true.