When I saw that picture the first time lo, these many years ago, I used a blue gun (rubber replica small-frame revolver) to try to figure it out.
Holding the gun as though to fire it at a target to one’s front, and then turning it to place the muzzle at or inside the mouth, unwieldy.
Holding it in the same manner but rotating the wrist so the gun is upside down, more “comfortable” (even with a dummy gun, my heart was racing throughout this procedure and I had to stop and rest from time to time).
Placing the thumb in the triggerguard and wrapping the fingers around the back of the handgrip, “best” and most “comfortable”, and the muzzle easiest to direct. Another test with a real gun, pointed in a safe direction, showed that the thumb (mine, anyway) is strong enough for a DA trigger pull.
Look at the photo.
Gun on RIGHT hand.
Unlikely to say the least.