It was an implied joke about academic pomposity towards those who are self-taught. Never mind...
I never felt the mentat-training aspect was that important in the books, although it would have helped Paul deal with all the weird stuff that was happening to him and his need to consider dispassionately what morally questionable actions were necessary to ensure the long-term survival of humanity. I can't remember if Paul in the book ever emotionally snapped at his mother about creating him as a monster. It makes sense to inform the viewers of the movie that it is not just some white saviour trope. Paul didn't succeed in his endeavour, despite his abilities. Leto II's solution was to become a god and eventually let himself be sacrificed.