This isn't a government study. It's a paper written by independent researchers in Germany that analyzed data from various sources.
Having said that, the hypothesis is intriguing and maybe should be studied.
Having adequate levels of D is always a good thing. I never knew that I was a little low until I read about checking the levels on this forum. My doc would never have checked without me asking .
Analyzing data of other studies can be problematic when drawing conclusions. We know that minorities in the United States have worse outcomes when they get Covid. We also know from studies that people with darker skin color on average have lower levels of vitamin D. We also know that they have higher rates of comorbidities. So what causes their Covid infections to be worse? Lower levels of vitamin D?
Taking vitamin D is safe but so is taking a Covid vaccine and we know DIRECTLY from studies that the vaccines are safe and very effective.