I think this might be a wisdom problem rather than intelligence. It sounds to me like something an immature paladin who spent his entire life in a temple reading about tieflings and never actually seen one would think and say
Wisdom, at least in the fifth edition, is also not very useful for paladins, well, unless they need a decent Insight score. In fact, most paladin builds only need high levels of charisma, strength, and constitution. Come to think of it, jokes about stupid paladins did not appear out of nowhere.
