Nobody claimed otherwise. The claim was that active Vitamin D receptors avoid developing the most pernicious effects of a COVID infection.
Most people infected with COVID that had to be hospitalized very likely had a blood test done, especially those sent to ICU, so it would be fairly trivial to look if the worst cases all happened to be Vitamin D deficient (or the vast majority of them).
A study showing that would give this theory more credence, and the fact that it hasn't materialized after 8 months makes it simply less credible.