Yesterday my (20 yo) kid was mountain biking with his friend, who crashed and was knocked out, stone cold. He had a full-face Giro Switchblade. The helmet was severely impacted and the jaw guard cracked. Despite a concussion, he got a clear CAT scan, and only spent about 7 or 8 hours in the ER, but I am fairly confident things would have been a whole lot worse had he had an inferior helmet, or even a good road helmet, let alone no helmet.
So add me to the uninformed random umpteen posters.
Unless one is willing to do the obvious experimental controls, I don't see how making such an observation is at all unreasonable.
Frankly, I don't understand why this assertion is at all controversial, and if for some reason it is wrong, so what? It is far better than being under-protected and being wrong.
He didn't have knee guards, and they spent several hours cleaning those wounds. It is possible if he had knee guards, it wouldn't have made a difference, but I think there is a reasonable chance it would have.