[AzNightmare]

What happens next in the video? You would be able to see if any passenger in the car at least turns back to look at the car or the driver behind who hit them. Natural human reaction.
Based on the video, doesn't look like there was a passenger... But as for solid evidence, there is nothing definitive that shows there wasn't a passenger.

Submit the video since there's nothing to lose. If there was a passenger in the car, they would have to be rather short. Investigation could press him to see who exactly was the passenger and maybe come up with a contridiction based on that person's height. Good luck with your situation.