Yup, more proper English for someone speaking toki pona would be tokiponist. However, this is a proper name and role title for someone in a fictional universe. The meaning of the total phrase is unrelated to or goes beyond someone speaking toki pona. And rules aren’t that strict for proper names. Even stronger, the friction is partly intentional. The shorter form for this character,Tipi, uses the syllable ti which is not even allowed in toki pona.