The Kara-Khitan at the time were Buddhists -- not Christian -- and there is no reason to suppose Yelü Dashi was ever called Prester John.
Os súbditos de Kara-Khitan nesa época eran budistas e non cristiáns, e non hai razón para supor que Yelü Dashi xamais fora chamado Preste Xoán.