The official guidance for the “not an answer” flag reads:
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.
Speaking as a moderator, I don't want users to use this flag if they think the answer should be converted to a comment. If you as a user have determined that the answer needs to be converted to a comment, you know which post the comment should go on. So tell me! Don't say “not an answer”, say “other”, “should be a comment on <link>” (or “should be a comment on the question” or “should be a comment on Bob's answer”).
“Should be a comment” is not a situation that can currently be handled by anybody other than a ♦ moderator. So there's no point in making this flag seen to users with the “moderator tools” (10k) privilege — either the post ends up deleted by users with the “trusted user” (20k) privilege (so it never gets converted to a comment), or the flag ends up being shown to a moderator, in which case it might as well be a custom flag.
I was surprised to hear from experienced users that NAA is the right way to request converting an answer to a comment — it contradicts both common sense, and my experience as a moderator. But I have to admit that the official guidance is unambiguously on their side.
I want to remove the “not an answer” flag altogether (replacing it by a clearer and more efficient workflow). But in the meantime, let us at least remove “a comment” from the official guidance.