It does not appear to have been peer reviewed. It is being hosted by a diet book author, which should be a red flag. The most reputable source they cite for correlating Vitamin D level with Covid-19 cases and deaths called the correlation "crude" [0]. It is a weak correlation. Their suggestion that there have been controlled studies on infected patients seems to be misleading. As far as I can tell from the sources they cited, none of them administered Vitamin D to Covid-19 patients. I think the trials they are citing were performed years ago on pneumonia patients, not on Covid-19 patients [1].

[0]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7202265/pdf/405...

[1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4939707/pdf/mai...

How do you evaluate the credibility of anything in life? I'm not being facetious, it's a question I've been asking myself a lot lately after making decisions based on the information from a medical doctor, who, turns out, was not competent to make certain affirmations.

There's other studies cited in this discussion. Those are peer-reviewed and do back up what the post is claiming.