I think this all stems from Baby boomers controlling the narrative. Baby boomers had an insane relationship with the sun. Getting crispy brown tan, using tanning oils, using that metal collar to blast sun directly into their face, and frequenting tanning beds were viewed as totally normal and healthy things.

Big surprise they all got skin cancer. Then they swung the pendulum all the way back and now preach 24/7 sunscreen and never letting the sun touch you.

The article doesn't make really controlled findings. There's an argument to be made that the increase to diseases isn't purely lack of vitamin D but lack of exercise.

Our ancestors got lots of Vit D but they also got lots of exercise while absorbing the sun.

I still don't think it's going to be wise to go out and just bake in the high UV of early afternoons but rather it's important to go outside early to mid morning or late afternoon and absorb some sun without copious amounts of sunblock.