Kleanbore...Time for my irritating penchant for Point/Counterpoint.
Don't take it personally.
My CCW instructor said "a hammer is a hook."
The same thing can be said of a rear sight or practically anything that protrudes from the gun.
Apparently your instructor doesn't understand how to draw a revolver with an exposed hammer from concealment. The thumb is correctly placed on the spur, acting as a shroud. It's worked since revolvers have been carried concealed. It also expedites cocking the hammer on a single-action revolver...which is precisely what was done. The gun was cocked before it ever cleared leather, and the finger entered the trigger guard as it was coming on target.
I can hear you say, Keep your finger off the trigger until you're ready to fire, and that's true, but we also know that people don't always do what they're supposed to do in the stress of a deadly encounter.
Indeed. The savvy gunmen understand that the practice of keeping the finger off the trigger until the decision is made to fire can get'em killed if their adversary knows a little about reaction times.
It's a point that I've demonstrated to several hig-speed/low drag types...and I mean the real McCoys, not the wannabes...much to their consternation.
I'll say it straight up, so there won't be any misunderstanding.
If I have a gun in one hand...pointed straight down at the ground, and you or anyone else points a weapon at me with finger alongside the trigger guard...and we're at standing apart at a distance of up to 30 feet...I'll shoot you before your finger hits the trigger.
I've been demonstrating this for several years to old and young alike...cops and Joe Citizens. Nobody has beaten me yet. There have been a few who were able to fire at the same time if their fingers were on the trigger...but they knew that I was going to move, and were anticipating it.
Please note that I'm old and slower than I once was...with a touch of bursitis in my shoulders. Someone young, athletic, and agile...with a little practice under their belts? Finger off trigger=a tag on your toe if you stand still while you face'em down. Guaranteed.