From the discussion section: "In acquired immunity pathways, 1,25(OH)2D has more modulating effect. 1,25(OH)2D inhibits activation of B-cells [29] and immunoglobulin synthesis [29]. This hormone also promotes Treg cells, which are responsible for anti-infectious action by inducing IL-10 production. This leads to suppression of Th1, and Th17 cells and IFNγ, IL-17, IL-6, IL-23 and IL-2 production and makes Th2 cells predominant. Th2 cells limits inflammatory processes by inhibiting Th1 cell-mediated cytokines and tumour necrosis factor α (TNFα) [11, 30, 31]."
Yes this could all be correlative, but lets assume for a moment that the effect is causal and as significant as this study is finding. It may seem like the effect of Vitamin D on the adaptive immune system doesn't make sense ... decreasing the antibody response and pushing the T-cell population towards a more TH2-type response. So, at least in terms of the adaptive immune system, Vitamin D is actually ~calming~ the immune system down, at least with regard to severe inflammation.
One way to make sense of this result would be as another piece of evidence pointing to some people's immune hyper-activation('cytokine storm') as one of the forces behind severe covid19 disease. This tends to be happening in the second and third week of infection and leads to much more devastating results on the body, and in particular, the lungs with ARDS (2).
(1) https://journals.plos.org/plosone/article?id=10.1371/journal... (2) https://link.springer.com/article/10.1007/s10067-020-05190-5