I'm sure not a scientist but since the study is focused on voles and voles typically aren't exposed to significant amounts of sunlight and its associated atmospheric radiations I think it possible the beneficial effects are illusory and simply compensate for a natural deficiency. I doubt the results could be extrapolated to demonstrate benefits to humans...unless of course they are trolls.
But if there truly is such a compensation, then doesn't that demonstrate that low levels of radiation are beneficial to at least one type of mammal? It is currently presumed that less radiation exposure is always better. This study seems to provide evidence to suggest that is not a universally valid claim.