Correct: fair use is a defense. Like "self-defense" or "coercion" are defenses in assault cases.
The affirm part is you conceding you committed the crime, and then you argue it was justified.
Lenz didnt confirm it was an affirmative defense, they ruled the way they did despite admitting it was an affirmative defense, one that can only procedurally arise after you've been accused of a crime.