Marine Cpl
.577 Tyrannosaur.
If you really cannot see how the thumbs prevent Full Contact compared to both thumbs forwards, I really do not know what to tell you. The pictures show it plain as day and if you grabbed your own gun, you will see how putting a thumb under a palm creates a space that is now not being supported.The weak hand palm is free to contact the grip. Thumbs aren't that big, and hands are free to conform and flex.
You say this thumb position makes the grip outdated. Most competitive shooters gloss over thumb position in their instruction. Some advocate thumbs forward to assist in pointing the gun. Nobody says thumbs forward aids in grip or recoil management. All advocate a high grip, maximizing the amount of skin on the grip. Thumbs position is of minimal, if any, consequence. Almost everybody relevant today is teaching essentially the same grip, with the biggest difference being the amount of "squeeze" applied - and that's all over the place from firm to tight.
You are defending this guy like he’s some family member of yours. There are other issues he teaches that are outdated or just plain wrong or just fear mongering as well.