I'll quote from the first paragraph of court's opinion:
We hold that the statute requires copyright holders to consider fair use before sending a takedown notification, and that failure to do so raises a triable issue as to whether the copyright holder formed a subjective good faith belief that the use was not authorized by law. [2]
Note that the case was about abuse of DMCA takedowns by copyright holders if not considering fair use. Judge considers fair use as authorized by law, which I would interpret as right, not defense.
Another quote:
We agree with the district court and hold that the statute unambiguously contemplates fair use as a use authorized by the law. [2]
Fair use is not just excused by the law, it is wholly authorized by the law [2]
From previously quoted wikipedia article, the paragraph continues:
"Fair use is therefore distinct from affirmative defenses where a use infringes a copyright, but there is no liability due to a valid excuse, e.g., misuse of a copyright." [1]
As for plaintiff considerations, IMO, of course plaintiff is required to argue it's standing, otherwise the case is thrown out, or in this case DMCA is abused. Normally the first bar is to make a coherent argument, then the court gets to decide whether your opinion is right, but that does not lower the defendant's rights to a "defense", even if they are a defense.
[2] Court's decision, hopefully: https://scholar.google.com/scholar_case?case=125676491686801...