This is the same as the teeth flossing skeptics: observational studies suggesting a paradox are interesting, but they actually need to unearth convincing alternative models with at least as much support as the dominant one for why they're right.
As mentioned in article, but also some general knowledge in physics is that interaction of radiation with the matter is probabilistic. It is not like every quantum of UV radiation will always damage DNA, which always result in cancer. There's some "conversion funnel" that depends on a number of parameters, and the actual probability of cancer in the end could indeed be lower than probability of the negative consequences of low sun exposure. Even if physical mechanism of diseases is established, it does not mean we should always avoid the risk by disabling it by some expensive means. Sometimes it is low enough to accept it or mitigate (e.g. by efficient cancer treatment).
That said, I'm not sure why this would be better
than just taking nitric oxide
The digestive system involves a pit of acid and a diverse, poorly-understood, and variable biome. It is an extremely fraught and indirect way to introduce substances into the body. Obviously many things can be introduced this way, but the list of those that can't is probably longer than the list of things that can.
It's ironic that the author didn't consider that getting sunlight may similarly be the side effect of the true cause of the stated benefits (being active?).
It's true there may be some important difference between sunlight-induced NO generation and whatever other supplement/injection/lotion/etc, but my point here is that the right approach is to focus on the mechanism, and avoid the naturalistic fallacy.
Wait, has there been a study suggesting this? This implies that there's a narrow optimal range and exceeding it causes problems. Seems you would see this in the supplement trials. Vitamin D toxicity doesn't occur until you take many times the RDA for months, and presumably none of the supplementation trials went anywhere near that high. https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...
I agree that just dropping a seemingly safe belief should take effort. I have no problem challenging them on a regular basis, though. Especially with more than just arguments and thought experiments. More observational studies can almost certainly only be a good thing.
Also, that UV damages DNA and causes cancer isn't simply a "proposed" mechanism, it's been tested many times.
I would say pure "observation" per se has little value -- for it to be scientifically valid it needs to seek explanations.
The mechanism isn't really at question. But just as sun exposure can kill grass, so to can lack of sun. We know that humans don't do photosynthesis, but it seems reasonable to ask if we do get other benefits.
Note. Is reasonable to ask and study. Is not reasonable to just assume the conclusion.