Soda is not thought to be healthy, but there is a difference between labeling something unhealthy and developing practical guidelines for its consumption. Someone who drinks 12 Cokes a day is clearly an outlier, but what about someone who drinks one? Should that person cut it out? Is there evidence to suggest one soda per day is harmful?
You are right, most whole grains are gluten-free, but the most commonly consumed whole grain in the country (not counting animals) is wheat, which most definitely has gluten.
I'm not arguing that nutrition is adequately covered in medical school, its treatment is
clearly variable. But it's a whole lot easier to point out a deficiency than it is to craft a durable solution.
The cited numbers were derived from a 2006 survey (abstract
here) that was trying to determine opinions about the value of nutrition counseling, not nutrition itself. My interpretation is that opinions about such counseling likely deteriorate because few patients are willing to actually make substantial changes to their diets,
which in turn feeds provider cynicism.