This is pretty good evidence that the limo slowed down but it did not come to a complete stop. Watch in the upper right of the Nix film. As the cyclist is trying to keep pace with the car and the head shot hits, he suddenly slams on his brake. You can tell because the shock absorber bounces as he stops.
The limo though never stops, just slows down then speeds up and lucky for Hill he was running at full trot to grab on.
FWIW - I've used this video ad nauseum to prove that the Z film is original and not doctored in any way. But of course the crazies out there always disagree with that assessment.