I saw the quoted text, but missed any explanation. I would love a clarification on whether they claim the skin is not getting exposed to sun or, even when expos

I saw the quoted text, but missed any explanation. I would love a clarification on whether they claim the skin is not getting exposed to sun or, even when exposed, does not make any vitamin D, which is how I read the above. This is an honest question -- if it is the first, I am not too concerned. I grew up in a colder climate and, while living far in the shaded area today, routinely wear short sleeves and walk a lot outside from early spring to late fall. If it is the second, I would love to learn the underlying causes at least as a scientific curiosity (and start thinking about vitamin D supplements).