Yes, the very same paper.

> Although results of an observational study, such as this one, need to be interpreted with caution, as done by the authors [1], due to the potential of residual confounding or reverse causality (i.e., vitamin D insufficiency resulting from poor health status at baseline rather than vice versa),

You need a control group with vitamin D deficiency but no other health issues to prove the hypothesis. They are just stating there's a correlation worth looking into.

Rather, a control group who does not take vitamin D supplements. Choose them randomly and you can find causation (or lack thereof).