Yeah, the wound was on his left thumb, so he was using a pistol-style offhand grip for a revolver, had his thumb so far forward that it crossed the cylinder gap and it got blasted.
This is the cylinder gap:
When you shoot a revolver, gases explode laterally out of that tiny gap, making a knife out of fire and pressure.
See where that left thumb is in relation to the blast? On automatic pistols you can put your offhand as far forward as you like, because there's no cylinder gap.
Try this grip with a revolver, however, and you'll get your thumb seriously hurt, as Tim demonstrated.
He was also wrong about putting his thumb in front of "the barrel" since that implies he stuck his hand out in front of the entire gun. He didn't. He was just using the wrong two-handed grip on the revolver because he started shooting one without the proper training.