Interesting, the linked paper [0] does show they controlled for physical activity here, since I would assume someone who gets more sun exposure, is outside more. If they really have controlled for that factor, shouldn't we see similar results from people with sedentary lives, but high UV exposure? (possibly by regular use of tanning beds or the like?) Being prescribed UV exposure is an interesting idea for the future…
As a general just-so-story rule of thumb to be discounted in the face of evidence to the contrary, stimulus or stress that's a consequence of doing stuff generally acts as a "growth factor" even if it causes slight damage, including exercise, heat and cold shock, and sunlight.