This why the author felt better and in general why most people do as well. Going on a diet that restricts a food group inherently steers you back to fresh, whole food that has not been processed. Processed food is what is killing people.
Four years ago, I detoxed and went to pure foods that aren't overprocessed. I didn't give up gluten, but limit it to a bowl of whole-wheat cereal and a slice of whole grain bread or some other pure grain. Without any added gluten, whey, cornsyrup, no artificial anything...and sticking to whole grain, two servings is fine.
I agree it's the processing. But I'd say it's also the additives.