Dude finds a fossilized turd and is ready to generalize about the whole “native-american” diet? Does he understand that there were hundreds of tribes and dozens of nations located all over the continent and the chances that they were all eating “incredibly fibrous diets” are rather slim? Has he considered the possibility that this brave had just eaten a tasteless, horrible meal that the wife of the big chief had gotten her hubby to force everyone to eat, and that the next day he went back to his mastodon steaks and bronto burgers?
He didn't say EVERYBODY!
BTW, the desert Southwest used to have a very thin, miniscule population due to the absence of enough water to support a large population. It wasn't like it is in modern times.
Or that the natural variation course of rainfall changed the sources of their diets? Or that when interactions with non-endogenous people exposed them to different bacteria, fungi and viruses? Or that sanitation conditions changed? Or that more sophisticated weapons made it easier to kill game? Or that the RESEARCHERS WERE ALREADY COMMITTED TO THEIR FINDINGS THAT DIETARY CHANGES WERE RESPONSIBLE AND ONLY EVIDENCE THAT WAS CONFIRMATORY WAS GIVEN ANY CREDENCE?
To name just a few.