Vitamin D deficiency entered the chat. It's a relatively common issue in many countries.

That's fair, but it also exactly explains why there are weak positive effects of extra Vitamin D.

There's a lot of unknowingly deficient people out there who get benefits from supplements. But the benefits are limited by the upper bound of the deficiency.

Two things to consider: The recommended levels are established based on "good enough for 95% of people". That means that quite a lot of people can get by with less than the recommendation. Furthermore, being deficient is not a binary. If you are just a little bit deficient you may have very mild symptoms.