That isn't a source for vitamin D supplements not having an effect on COVID, that's a source saying that sunlight is needed to produce nitric oxide.
Looking online, I see that there is a hypothesis that nitric oxide has a protective effect against COVID, but I don't see that this hypothesis is any better tested than the hypothesis that vitamin D itself has a protective effect against COVID.
So, sunlight will probably get you the best of both worlds, as it would help either way, but there doesn't seem to be real evidence for nitric oxide vs vitamin D.
Yeah I would describe the evidence for both as circumstantial but fairly compelling, and very low risk. Even given the unknowns and potential risks, getting enough sun exposure is probably the best anti-Covid intervention from a cost-benefit perspective.
Looking online, I see that there is a hypothesis that nitric oxide has a protective effect against COVID, but I don't see that this hypothesis is any better tested than the hypothesis that vitamin D itself has a protective effect against COVID.
So, sunlight will probably get you the best of both worlds, as it would help either way, but there doesn't seem to be real evidence for nitric oxide vs vitamin D.