> From the perspective of your immune system's cells, what exactly does the vitamin D molecule do?

Probably nothing. High vitamin D levels are a side effect of being outdoors a lot and therefore getting more exercise than the deskbound population. I recall during lockdown listening to a Slate Star Codex article (on the Astral Codex Ten podcast) which went through a stack of papers and concluded that there was nothing to see here.

The article should be retitled "Pre-Infection exercise levels associated with severity of Covid-19 illness".