Disagreement about the tenets of healthism is not just about its dogma of perfect diet. It's also about terms of reference. There isn't agreement about the definition of health-i.e. how healthy does anyone need to be over and above the needs of their existence, way of life, personal tastes and inclinations?
We know that if your food is short of necessary macro and/or micro nutrients, it's likely to undermine your health and cause disease in the near, medium and long term future. What we don't know is if you have a balance of those and enough to eat, that there's some optimum level of micro nutrients that can elevate you and your health to a higher plane. There is an element of mysticism about the healthist mentality which can be very uplifting and seductive, especially to a perfectionist mindset.
Healthism can see food in an almost spiritual way. It underplays, sometimes to the point of disregarding humans primary need, energy as if that is base and vulgar, to focus more on high faluting things like micro vitamins and minerals. This has helped to skewer our understanding of what the purpose of eating is. Which makes it rather regressive in nature. Eating is built around seeking enough energy, with energy dense foods at the core.
It's also not clear whether this extent of direction of diet itself causes imbalance in hunger and appetite and is itself unbalanced. Whether it short changes by default, i.e. the avoidance of calorie dense foods. Or whether the sources of certain nutrients it favours, lead to an overall imbalance by excluding other foods.
Too few people seem to follow it to the extremes, perhaps. Or maybe they're not the ones studied. It is very hard to quantify and classify the quality of a diet, over and above the basic requirements of say a meal featuring protein, high water veg, denser starchy veg with perhaps some fruit.