Im just guessing here but but his patients come to him from all over the country and he tests them all for Iodine sifficiency and out of the thousands tested 95% have been shown to be deficient.
Maybe not having tested every citizen of the world stating 95% of the population may be a stretch BUT I think having tested thousands of patients its fairly safe and accurate to extrapolate that and include "95% of the population" as a fair estimation.
Having tested thousands of people is a LOT more inclusive than many studies/papers whose claims are beleived by a vast majority of the medical profession.