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.

My understanding is that it’s extremely hard to get large scale controlled studies on sun exposure vs vitimin d supplementation, so there is lots of uncertainty still on relative efficacy. I wouldn’t say certainly that it’s inferior to sun exposure, but sun exposure can clearly have many more effects than just stimulating production of vitimin d that could then appear to correlate with D serum levels.

Some more reading specifically on vitimin D:

https://ods.od.nih.gov/factsheets/VitaminD-health%20Professi...

I have heard that sunlight also causes the skin to produce nitric oxide, which supposedly has all kinds of benefits.

I am not claiming this and don't have a reliable source to point to.