As I age, I increasingly see the effect that healthy people with great genetics also have all the indicators of being healthy. I think this effect is conflated to an extent even for things like exercise: for people that feel good and strong, exercise feels great. For people that aren't, they go downhill faster anyway.
I have terrible genetics for cardiovascular disease. I've spent my life as an athlete, eating healthy, training, etc. But now in my 30's, I can feel the slow down a lot despite being more disciplined in my training and diet than ever and yet I know people my age that haven't trained in years that can just go for a run and absolutely smoke me.
I'm sure at some point my genetics is going to force me to slow down a bit on the training despite my best efforts and then I'll just end up as someone contributing to the statistics that those who exercise more are healthier.
I know that exercise has benefits nevertheless, but I also think we very often just make the mistake of concluding "healthier people are healthier".
Life is not really fair.
most cells have vitamin D receptors, so I would think it is actually a driver
That is not true. AFAIK there have been several studies using supplements of Vitamin D3 & Omega-3 that showed that it improved biomarkers of various diseases.
Similar significant results for symptoms of cystic fibrosis: https://pubmed.ncbi.nlm.nih.gov/32275788/
Some vitamin D skeptics claim tat there's insufficient evidence from RCTs but few people question the causality of smoking's influence on cancer despite no RCTs for that---that relationship was established also via Hill's Criteria.
Both my statement and yours can be true. Having a deficiency in an essential nutrient can be causal for bad outcomes while, at the same time, having the correct amount or excess can be non-causal but correlated with positive health outcomes.