The flag declination was absolutely appropriate.
This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether
That is the flag reason you chose, and it's incorrect. The answer does attempt to answer the question. It may not be a high-quality answer, but it is an answer.
Flags should not be used to indicate low-quality and/or incorrect answers. That's what the downvote arrow is for.