Where is the data here? That article simply declares that there is a global epidemic of vitamin D deficiency and I followed the citation it gives to back that up, which is this: https://www.sciencedirect.com/science/article/pii/S000291652.... Except this citation is also not a study. It's an opinion piece that just declares people are vitamin D deficient without doing any measurements of how much vitamin D anyway has circulating in their blood. It appears to be a narrative review, not a study.

I'm not saying the claim is for sure wrong, but at some point, to make this claim, someone has to have actually measured vitamin D levels in some broadly representative sample of humans and you could post that instead of whatever came up first in Google Scholar when search for "vitamin D deficiency."