The author of the article is a science writer, not a scientist, expressing pure opinion with that statement.
People try pure vegan diets, but I have yet to hear of someone sticking with one permanently. There also isn't any research establishing that such diets are actually healthy. Nor is there any scientific reason to believe that vegetarianism is somehow better for the environment. I know that there is a lot of hype claiming scientific support for these assertions, but reading the original science articles reveals that the scientific basis isn't there.
I certainly don’t advocate anyone start eating large amounts of fatty meat every day, but the food nazis can drive a person over the edge. I’ve heard a few claim humans didn’t start eating meat until about ten thousand years ago because of some huge food crisis. I asked one young veggietarian who told me that story why humans have incisors and not all molars. He had no answer.