The previous observations can be dissmissed as isolated cases on the other hand Grossman cites a bunch of more throughout his book and refers to his conversations with lots of veterans with similar experiences like this tunnel rat's memories:
Then I cautiously raised the upper half of my body into the tunnel until I was lying flat on my stomach. When I felt comfortable, I placed my Smith Wesson .38-caliber snub-nose (sent to me by my father for tunnel work) beside the flashlight and switched on the light, illuminating the tunnel.
There, not more than 15 feet away, sat a Viet Cong eating a handful of rice from a pouch on his lap. We looked at each other for what seemed to be an eternity, but in fact was probably only a few seconds.
Maybe it was the surprise of actually finding someone else there, or maybe it was just the absolute innocence of the situation, but neither one of us reacted.
After a moment, he put his pouch of rice on the floor of the tunnel beside him, turned his back to me and slowly started crawling away. I, in turn, switched off my flashlight, before slipping back into the lower tunnel and making my way back to the entrance. About 20 minutes later, we received word that another squad had killed a VC emerging from a tunnel 500 meters away. I never doubted who that VC was. To this day, I firmly believe that grunt and I could have ended the war sooner over a beer in Saigon than Henry Kissinger ever could by attending the peace talks.

Or.
Colonel Milton Mater served as an infantry company commander in World War II and relates several World War II experiences that strongly support Marshall's observations. Mater also provides us with several instances in which World War I veterans warned him to expect that there would be many nonfirers in combat.
When he first joined the service in 1933, Mater asked his uncle, a veteran of World War I, about his combat experience. "I was amazed to find that the experience foremost in his mind was 'draftees who wouldn't shoot.' He expressed it something like this: 'They thought if they didn't shoot at the Germans, the Germans wouldn't shoot at them.'"

Of course all this still can be considered as anecdotal evidence.