The point is that Aristotle's views about Oedipus both misread Sophocles' text and fail to generalize across the literary corpus Aristotle was analyzing. Achilles is the quintessential and paradigmatic tragic figure in ancient Greek literature: if Aristotle's insight about tragic character doesn't apply to Achilles, then it certainly makes sense that Aristotle would omit discussing him because such a counterexample so discredits his idea!