Interestingly, you can see the marked police car pull up right behind, during the incident. Is there any chance that this car had a dashboard camera (with video and audio)? If a person stopped in the middle of a public road has a reasonable expectation to privacy, then dashboard cams could NEVER be used. In this case, even if the marked car's dashboard cam was not recording at the time, the unmarked cop would have known of the possibility (consent?), and thus would not have had a "reasonable expectation of privacy."
I know a hypothetical camera is not the same as a real one. But it would be very interesting to know if/how that would fit into this scenario.