Very interesting post and link Squiffy !! I agree with the study that there may be more to the sun's UVA/UVB rays interacting with our skin cells than to just produce vitamin D as they mentioned certain compounds being created when exposed to the sun however what about certain epigenetic expressions only being turned on/off by the suns rays.
Here's something i studied off subject from MS but non the less it taught me about the UVA and gene expression as although i'm caucasion and we are from england paternally and italy maternally my white skin turns very dark when exposed to the sun and so i researched as to why that happens and what i learned is that we all have a gene in our skin cells called the ''p53 gene'' but in certain humans when the sun's UVA rays strike the skin it activates this gene to tan the skin as a protector and it has nothing to do with vitamin D production.
So in other words maybe getting little or no sunlight on our skin may give us these autoimmune disorders not by low amounts of vitamin D but by not activating certain epigenetic expressions that keep us well and healthy as there could be hundreds or even thousands of genes turned on and/or off when exposed to the sun's UVA/UVB rays.
By the way while i tanned dark as a kid and teen as i always played outdoors in sports for some twenty years after i hid from the sun as i was told my my fitness and health magazinesback in the 1980's and 1990's that it caused skin cancer and premature wrinkling and so i never tanned again for years and years and well i've come down with MS and so hmm ...... ???