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.
So there's no point in making this flag seen to users with the “moderator tools” (10k) privilege
very true. One could simply add "To request conversion to a comment, choose ... below" to the guidance