Yes but saying you can't go from the glass end of the spectrum all the way to iron on the other end is a bit silly, just unnecessarily stating the obvious as much as it would be to say that Paulie Malignaggi could ever do anything to become a bigger welterweight puncher than Felix Trinidad...or that Carlos Maussa could ever become faster than Meldrick Taylor. An extremely terrible chin, however, can be conditioned to be less terrible, no?
This is logical. Concussions are the result of rapid deceleration of the head causing the brain to slosh up against the side of the skull. Lean your head back and look up at the ceiling and then try to turn your head as far as it will go side-to-side. Then try putting your chin to your chest and turning your head side-to-side. You can't turn your head as far the higher you raise your chin, which would cause rapid deceleration of your head. Not only does keeping your chin down protect it from being hit in the first place; it also allows the muscles of your neck to help prevent rapid deceleration of your head after being hit. I'm really just talking out of my ass here.
I do agree with the mental aspect you mentioned. A lot of things in life is mental. Aaron Pryor is one guy that has talked about how he "mentally wasn't going anywhere" when he was hit with a big shot and he would just "eat" them (his words) The whole training thing to me would be maybe a 5% difference in my opinion not much. How many Glass Jawed fighters have we seen claim they were doing neck exercises and squats etc to help with their punch resistance and it did nothing. The list is long. Lets go back to the Khan example Hunter had him doing the neck exercises and he still was put down by a feather fisted opponent. The thing that helps fighters like Khan is fighting opponents with grossly low KO rates. Which conversely is exactly what Roach told him to do when he left him for Hunter.
Look how Floyd took that big right hand from Maidana. He has a good chin but without neck work that punch would have taken his head off
Its more to do with position and your style of fighting than neck muscles. Juan Urango a massive 140 fighter was knocked out by a pretty light hitting Alexander.
Around 75% of a punch's power get absorbed over the neck, so yes having a strong and muscular neck helps enormously. If you do combat sport and happen to get clipped, you will realize this the next day.
Eight thousand, huh? Funny, that's a couple of millennia further back than recorded history even goes. We know wrestling is that old from cave paintings, but I somehow doubt there are specific cave paintings of the neck exercises, if any, done in conjunction with ancient wrestling before recorded history.