He was reacting to alerts, complying to them by simply touching the steering wheel. He did that 150 times during that 45 minute trip ( not all the trip was on auto pilot).
So if the guy died the car would of disengaged auto pilot (I'm not sure how this works).
You can check the video in the article. It's quite informative .
Edit
I saw another video and it takes ~60 seconds after taking off your hand from the steering wheel for the car to safely come to a full stop.