It's possible to investigate whether diets produce beneficial results in subgroups even without having a proper general model of diet and nutrition.

It's analogous to investigating whether drugs produce beneficial results in people with X disease even without having a proper model of how exactly X disease works.

That also makes no sense to me.

It's like saying you need general recommendations for sun exposure before you can research specific recommendations based on skin type/ethnicities.

It's nonsense. Pale-skinned Caucasions need different recommendations from other ethnic groups with more baseline melanin.

We don't know that pale-skinned Caucasians need different recommendations in this scenario. That's the point. We barely know anything about nutritional science, let alone what one specific race of people needs.

If you have no clue how sun exposure actually affects the body beyond "ow burn hurt", you don't have the necessary knowledge to begin creating highly specific treatments. What would you base the science on? There's no real foundation.