Note that I have asked this same question at English.SE and consider the question here to be a continuation of Let's bring down the number of unanswered questions.
ELL users have the habit of answering questions in comments. I've seen a lot of them over the time I've browsed this stack, the examples from CowperKettle, cited above, are just two examples that I ran across today. I've seen it done by new users and veteran users alike (more at English.SE than here, but that might be a subjective observation).
It is the natural habit to not answer a question when we see that someone has answered the question in a comment. Most especially when the comment is fundamentally correct. We don't want to steal the thunder, so to speak, of the answer — even though the answer isn't an answer but a comment.
What is the preferred behavior when other users find these pseudo-answers that can't be downvoted, edited, flagged for quality by the community, or commented upon without cluttering up the question comment chain? Should we flag them for moderator attention (or another reason)? or should we ignore them?