> This says nothing about Vitamin D as a cure once you got infected.
I mean, yeah. There is literally nothing in the study, or literally any of the research I've read, that says anything about vitamin D being a benefit after you get infected, so I'm not sure why one would appear to think that's implied. It's like saying "Hey, this study isn't as strong as you think, because this study says nothing about vitamin D increasing your IQ."
The "correlation is not causation!!" bromide, while useful to understand in context, is overused, often given with examples that are more likely to be outlandish, with no backing evidence, than with a more straightforward explanation. As others have commented, there is evidence that vitamin D itself has a protective effect.