That is not true, he avoided that term. He believed that demons could have an influence on the human body but not demon possession. So that is not the case, he never once said a person could be demon possessed. He has given accounts of why he does not use the term demon possession.
He has a brilliant way of explaining his teachings, keeping them simplified.
Possession means having total control over something, owning it, Derek Prince did not believe that. He believed in demon influence over someone.