I doubt anyone is saying with 100% assurance this is the -cause- but that it would be worth investigating further. If they'll change FDA recommendations for the blowhard in chief, we should also consider lack of certain nutrients as possibly something to look into as well.
I recall an article a few months ago that discussed the Vitamin-D link in more depth, and a plausible mechanism was already suggested: vitamin D regulates the immune system response. People with low vitamin D levels are more likely to experience severe cytokine storms.
The impact of Vit D in enhancing immune response (including flu and previous coronaviruses) has widely been studied and firmly established. Vit D can also suppress cytokine storm, which may substantially increase the chances of avoiding a severe case of COVID-19