Aytu BioScience is working on an ultraviolet device to kill viruses in vivo. Initial results look promising but it's too early to say whether that would be an effective therapy.

https://aytubio.com/healight/

This one was also infamously censored by social media this year.

By "it" what are you referring to?

Vitamin D may have modulatory effects on the biochemical pathways linked in the original article. Most pharmaceutical products we study aren't going to directly kill bacteria or viruses, usually they have a particular physical effect on a specific molecule that then leads, through various cellular pathways, to the death or suppression of the pathogen. I highly doubt Vitamin D directly kills anything, it could modulate symptoms or the pathophysiology of COVID-19 the disease, vs. SARS-COV-2 the virus itself.

If "it" means UV light, yes there are UV sterilizers aplenty today. If "it" means outdoors/fresh air, there are a lot of components to that and it's not like "fresh air" enters "in vivo" per se.

I mean UV light/being outdoors

Yes, I'm aware of UV sterilizer, but does it work when the microorganisms are inside you? You don't want to get an "internal tan"

If it produces better outcomes in vivo, does it matter whether or not the proposed mechanism is correct?

Ideally you'd like to understand the mechanism so that you can isolate and optimize the the effect, as well as predict possible interactions/side effects.

Ideally. In reality it rarely happens. And it's certainly not needed, the entire basis upon which evidence-based medicine is founded is that an effective intervention should be statistically measurable in it's effect regardless of our knowledge of the underlying mechanics.