If you step through the video with the "," and "." keys, you'll see that the steering wheel ends up in contact with the drivers seatback. A seatbelt that holds the driver in place doesn't help if there is no safe space for a body.
The 59 GM Full Sizes with an X frame did particularly poorly in overlap crashes - it would have performed better with a full head on - or a slimly later one with a permitter frame.
It's also a more likely type of crash to occur. Not many cases where one would square off the hit straight on, I'd imagine rather most cases at least one occupant is trying to avoid the crash by moving to the right.
