I think the vehicle in this accident does have a user interface problem but as foreigner in the USA, I have observed that most Americans never use the parking brake or hand brake as I call it. Sometimes where I have lunch at a cafe and I sit looking out a window at a parking lot, I see lots of cars arriving. Pretty much without exception I see the car rocking back and forward as people get out or, if it's on a slope, it moves abruptly when the driver releases the foot brake and it crunches up tight on the transmission. I asked a few friends why they don't use the brake and I've only received blank looks.
I surprised that the warnings I've read about this accident are mostly just saying "double check that it's in park". I have seen a clear recommendation to put the brake on.