Obesity lowers serum concentrations of Vitamin-D. We know obesity is strongly correlated with severe covid. So not really that interesting of a correlation IMO.
But your criticism doesn't necessarily check out as the average BMI of group B was actually lower than that of group A.
See page 4 of the PDF [1].
People have known about this correlation for months now. If it were really a thing, clinicians would be publishing case studies like crazy of successful treatment with vitamin-D. I'm not seeing that in the literature... so I doubt it will pan out.
That's why the MATH+ protocol [1] recommends Calcifediol or Cholecalciferol. According to their own review [2] the protocol reduces hospital mortality from 20-30% to about 5%.
[1]: https://covid19criticalcare.com/math-hospital-treatment/pdf-...
[2]: https://covid19criticalcare.com/math-hospital-treatment/scie...