Just because a question is closed does not mean we should stop deleting crap answers from it. A question closure does not necessarily mean the question will be deleted, and we should still clean up any answers that don't belong, no matter what.
The answer you reviewed is just horrible. It doesn't answer the question at all and shouldn't exist. Your response should always be to recommend deletion of that answer, whether the question is closed or not.
Sarcastic, crappy answers to bad questions only encourage other crappy answers and discourage the OP from attempting to improve their question to a point where it could be reopened. In this case, it was closed as a duplicate and will likely never be deleted, but serve as a signpost. Even more reason to get rid of the answer.