There is very good evidence that vitamin D in general helps your immune response. It may be that there is a second factor here (another commenter suggested that UVA light was also involved in this process), but the evidence in favor of boosting your immune response by getting sunlight every day is pretty conclusive.
I would be surprised if this study was a vindication of vitamin D, though, rather than a false positive. I think smoking has been shown to generally have negative effects on your overall health (and immune system).
> generally have negative effects on your overall health
smoking tobacco for years, every day, definitely changes your body. Most of those effects are not great in the long run, yet many people start and continue to smoke cigarettes for years on end. The change to lung tissue, mucus membranes, tongue, mouth ecosystem.. are real.. and for an otherwise healthy person, yes, they have to get "tougher".
Vitamin D is obviously necessary, mostly for bone health, but any study of supplementing it for other purposes to people without any symptoms of deficiency have been disappointing.
I would be surprised if this study was a vindication of vitamin D, though, rather than a false positive. I think smoking has been shown to generally have negative effects on your overall health (and immune system).