It sounds like the tech support agent was trying to get him to do just that on his phone... But I haven't used Waymo before, so I'm not certain about that.
I’m guessing the “open the door and the car will stop” system was built by the same people who built the “get in this car and it’ll go to your destination” system.
What is your point? If he didn't try in the video, you can't say that wouldn't have worked. In my own experience, it stops every time the door is open.