A person with cross-eye dominance would have that problem whether shooting a light or a heavy gun. I think it is obvious that Stan was speaking of a person without severe physical handicap, like a missing arm or hand or leg or shooting in a back brace.
Saying that there are guys that can't shoot a light gun well is not the same as saying that light guns can't be shot well.
I have no doubt that a person without serious physical handicap could shoot a couple hundred rounds (5000 targets) of low gun skeet with one of my 5-1/2 pound small bore guns, 3" of drop, and work up to shooting 25 straight now and then.
Saying that practice doesn't generally improve performance is just wrong, whether we are talking about golf, tennis, hockey, piano, darts or shooting targets with heavy guns or shooting birds with light guns or vice versa. I didn't see where Stan argued that practice would turn anybody into a George Digweed caliber of shooter nor did I see where he claimed to be in Digweed's class.
