I understand that users with a low reputation are not allowed to comment other people's posts in order to prevent spam, since comments do not bring the posts on the top of the Recently active tab and therefore benefit from less reviews.
I understand too that, in theory, users with such low rep have the possibility, if they have no other choice, to post they comment as a new answer so it gets reviewed and if it is legit manually transformed into a comment by a moderator.
I notice that the flag meant to raise moderator's attention toward such posts happily mixes in a single bag valid and useful posts which should be kept and transformed into comments and posts which should be deleted:
not an answer: 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.
When such a flag is raised, the post is added to the Low quality posts lists where one has basically the choice between "Looks OK" which is not right because it is not a valid answer, and "Recommend deletion" which is not right too because the content is valid and should be kept (as a comment),
The meaning of this flag seems to be badly perceived by the reviewers, by the past I raised a few flags to signal such posts needing to be turned into comment, I think such flags were systematically disputed because these posts were not meant to be deleted while the post itself was effectively turned into a comment (I see this as a direct consequence of the previous point).
The automatic comment left when using this flag always seems a bit rude to me.
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post.
When I read this from a newcomer perspective (which should be the primary target of this message), I see "This is not a valid answer, use comments instead, too sad you cannot!", which seems a bit rude even-more when the poster already apologized of not being able to post a comment as I often see in such circumstances, and I understand some may take this wrongly (link useful only until the Nazi related comment gets deleted...).
Why isn't there two different flags?
One for actual low quality posts which should indeed be either edited or deleted,
Another for comments posts which should be transformed as a comment?
I think this would greatly clarify the handling of such situations, and accompanied with a more productive automatic message (why not link to a canonical explanation why newcomers are not allowed to comment? Wouldn't this make them better understand the situation, what is going to happen, and how this site has been thought? As newcomers, they may not know meta and the help center, and deserve some friendliness to encourage them to stay.).