If I understood it correctly, the man was so busy trying to control the Prius that he either didn't have a hand available to let go or he didn't think of it in time. Later, he said that he was afraid that if he put it into neutral the car would flip.

I also don't understand why he didn't turn off the ignition - which he did do when his speed was brought down to 55 mph- unless, again, he was so busy just hanging on that he had literally no time to think anything else.

In any event it was a horrendous scenario- amazing that he hit nobody.