Can't find it now, but this is probably based on a study/studies that I've seen that showed that lack of vitamin D is correlated with bad things, but supplementation doesn't fix the bad things nearly as much as you would expect. So, the idea some people have is that vitamin D is merely a correlate of something else that sun exposure does. I call it "sun magic" :)

...sometimes I secretly wonder if sunscreen (over-)use in America would one day be found to be the cause of rise in depression and other things like that over the last few decades, kinda like lead pipes of old. I don't think it's likely but it would be kinda funny.