Lere
You are talking apples and oranges. You are correct that sunscreen use won't affect the current serum concentrations of concentrations of vitamin D. That's not the point. That level of vitamin D is already in the system.

What sunscreen does is prevent up to 95% of vitamin D generation through the skin. And that means though you have vitamin D in your system, you won't add to it or replace the amount you use up which leads to depletion and deficiency.

Ah, and the use of suncreen did decrease 25-hydroxvitamin D levels...just not enough to induce secondary hyperparthyroidism.
Those of us here know that levels required to ensure parathyroid function are very low and easily corrected...much higher levels are required for preventive and optimum health.

And regards the nursing home residents...do you have that cite?

I'm assuming if they were 100+, they grew up in a much less modern era and were probably vitamin D replete growing up. Add to that organic produce, physical labor, outside work, Mediterranean environment, etc. and I'm not surprised at their longevity and health.