Is it believable that Khan could be highly intelligent yet make basic mistakes in three-dimensional space combat?
Short answerYes; strategic ability does not automatically confer tactical expertise, and Khan's certainty that he is the smartest person in the room compounds the gap.
NotesHighly intelligent people can excel at organizing a war while remaining poor at the immediate mechanics of combat. Strategy and tactical command are specialties, and genuine mastery of both is uncommon. Khan's deeper weakness is overconfidence: he assumes he is the smartest person present and fails to ask whether an opponent may understand the situation nearly as well. Expertise can become a vulnerability when status or intellect discourages self-questioning.