That said, I'm not sure why this would be better 
   than just taking nitric oxide

One might do well to start with the assumption that "taking XYZ orally" is not as effective as producing it naturally.

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.

Well, I never said "orally." If it's beneficial to get NO via sunlight on your skin, it doesn't seem crazy that it might work via some lotion. Even considering oral supplements, one might also take a precursor if the molecule itself isn't stable in the stomach.

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.

Surely you'd need to smear sunlight lotion on your skin if you want to use the existing the dermal route?

Not to mention that one of the premises of the article was that vitamin d levels were a marker, not some singular responsible agent.

The conclusion that vitamin D levels are an effect not a cause came from exploring the mechanisms. And yes, it may turn out that NO similarly is a marker.

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 may be, but NO itself is also very much a vasodilator, and used therapeutically for that.

One reason getting these from the evolved route may be better is because feedback mechanisms stop making the beneficial substance when the body has enough, which may not happen via intake by other methods. This is the case for vitamin D from sun vs oral D supplements. Serum levels stop going up when they reach a natural adequate evolutionary range if it's coming from sun. I don't know if that's true of NO or not. Or of melatonin from the near-infra-red wavelengths, but I wouldn't be surprised.

> This is the case for vitamin D from sun vs oral D supplements.

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...