Thanks for pointing that out, there have been major issues with drawing overly-broad conclusions based on studies pertaining to a specific demographic, before studying the general population.

For example, omega-3 fish oil became one of the most popular supplements since a 1970's Danish study showing it helped prevent heart attacks ... among the Inuit, whom it turns out are the only group that produce the enzymes needed to metabolize fish oil.[1]

That said this is far from the first or only study showing health benefits to vitamin D supplementation.

[1] https://www.nytimes.com/2015/09/22/science/inuit-study-adds-...