Yes, it is the spine tilt, and the tilted shoulder turn that flatten the barrel path out. His hands start it directly down and the body action and angles, flatten the path.
That's why telling someone to keep the back shoulder up and stay tall, will not work. If you take the barrel down while doing that, the barrel will go straight down across the ball's path and never level out.
Not true. If they literally stand straight up and stay tall on the back side. Then yes, they will be chop down through the path of the ball.
The stay tall on the back side is more of a feel thing. It keeps a strong backside that allows you to attack from above instead of below the ball. It tightens up the path of the barrel. It helps with creating a short, quick, direct, tight barrel path.
Jim, you have been around and if you have seen MLB players train, you know that they are thinking tall backside and stay on top. You should also know why those cues work.