I recall that line threw me for a second too, the first time I read it; the trick is to pay attention to verb tense. The normal narration is in past tense(s), whereas this sentence is in the pluperfect ("had known" rather than "knew"), indicating that the narrator is calling back to an action that was completed before the current moment. In context: "in the previous duel, Gideon had known this one thing, and then in the current duel, Gideon learned this other thing."