Secrets
Lot's of information to be learned here, in regards to what context he is correct and where he is wrong. Certainly one thing is clear - having an M.D. after your name or golf trophies on your bookshelf, or a PGA tour card in your pocket- doesn't make you right - as has been inferred here. If you learn that fact alone- you're a step ahead.
As far as "my secret" - the Spine Angle concept may have well been a KEY component that impacted his golf swing and scoring results more so than any other idea or component.
To imply if he does- that this is the secret for every golfer - or that this is the only thing or greatest thing- probably not.
He makes a nice psychological distinction between those that focus on "head down" versus "spine angle". Notice, in the video- "Head Down" would not mean head still - rather eyes down or chin down. He also finds for himself that the 'KEY" to maintaining the Spine Angle is an awareness of the shoulder turn. Notice that initially he doesn't let the "over the top" feel, stop him from working on that or understanding that move. He is aware that if he comes too much from the inside on the downswing that it would make him come out of his spine angle. So he has a very good understanding of some "seems as if" issues and how they impact his swing. Notice, as good as he was before 1992 that he was trying to "copy" Jack Nicklaus.
In summary, it's a swing change that he made that left a significant impact on not only his golf swing but his psyche. Discovering that "good things" like swing in to out- might not be good. How swinging from the inside can cause you to "come out of your spine angle". How the feel of over the top- might actually be a good thing- at certain times.
Does the plane of the backstroke shoulder turn match his downstroke shoulder turn- no. But that doesn't mean that the concept isn't helpful for him in relation to and keeping him from coming too much inside out.
Finally, just for fun - if you ignored age and all other factors except for when he learned "my secret" in 1992 and you looked at his Masters record (won it in 1981) - then you'd say that this "secret" made him a lot worse after it than before it. And to what degree and timing that the golf instruction "culture" was moving away from a reverse C and to a more "released", "level" finish- affecting or supported his "new" move- might have had some influence.
Many more things to learn from that clip but that's a few of them.