Intelligence (which psychologists define as the g factor [1]; this concept is very well-researched) does not make you an expert on any given topic. It just, for example, typically enables you to learn new topics faster, and lets you see connections between topics.
If Karpathy did not spend a serious effort of learning to get a good understanding of people, it's likely that he is not an expert on this topic (which I guess basically nobody would expect).
Also, while being a rationalist very likely requires you to be rather intelligent, only a (I guess rather small) fraction of highly intelligent people are rationalists.