The words "prevent" and "reduce risk" to me are confusing. Prevent is obvious but to reduce the risk would that mean ifyou have an autoimmune disease now it reduces health effects?
Vitamin D appears to be essential for immune system function and autoimmune disease is dysfunction of the immune system.
The article has a population point of view and risk means the ensemble probability of a given sample of people having said disease. It’s correct that risk is confusing from a single person point of view where probabilities do not make sense.