Did you read that link before posting it? It says that dietary supplements are of no benefit to people who eat a healthy diet that is providing all of their nutritional needs.
It does not say that people who are deficient in the nutrients should not take supplements, to be specific it does not say that people who are deficient in Vitamin D should not take Vitamin D as called for by Dolores Cahill.
Vitamin D insufficiency is not confined to "at-risk groups" and is likely to be prevalent in the general public in Ireland, especially in wintertime.pubmed.ncbi.nlm.nih.gov
Vitamin d supplementation is good for you. It does not cure or prevent covid