Mas Ayoob teaches strong hand thumb down.
My instructor, John Farmum teaches thumbs up.
A former Secret Service Federal firearms instructor, then with the NRA, didn't correct the different grips we used on a variety of pistols. Though he did with a couple revolver shooters.
I've come to use thumbs up--the thumbs stabilize the firearm in that position as they do in the thumbs forward, wrist down position.
"Proper grip for shooting an autoloading pistol" is a subjective term, and I don't think there is any such thing as THE proper grip that's universal, anymore than the "proper shooting position".
Use that grip pictured, and you'll end up activating your safety and deactivating your gun on the 1911. As long as thumb isn't under the safety unless thumb is down.Thumb ON or UP behind the safety works well on 1911, and other guns as well.
And changing the grip from one gun to another isn't practical as I see it. I assume, of course, that most of us own pistols with different safety systems and ergonomics. Strictly a Glock (or similar gun) shooter will be served ok with the grip pictured. I understand it's very popular with the gamesman
Just my thoughts on the matter.