You are correct that this comment is often used wrong. Our policy is that answers that are low quality, incomplete, or too short should not be posted as comments. It is sometimes appropriate to delete such posts, but they should not be comments.
Since we have a strong community consensus that these don't make appropriate comments, reviewers should not advise people to post them as comments.
We also have the help page on comments, handed down to us by Stack Exchange, which lists the situations in which comments are or are not appropriate. This can provide helpful guidance while reviewing, to decide whether or not to advise someone that their answer is really a comment.
Posts that are very low quality should still be flagged and should still be deleted in review, but we should only suggest an author leave a comment when it would not be wrong for the author to do so.
Furthermore, aside from the problem of giving newcomers bad advice about how to use the site, people often use that particular review comment in contexts where it does not make any sense. It says:
To critique or request clarification from an author, leave a comment below their post...
So if a post is neither an attempt to critique something else that has been posted, nor an attempt to request clarification on something else that has been posted, then this review comment makes no sense for it and should not be used.
This relates to another, broader problem. For some reason, many reviewers think the canned comments on the Recommend Deletion / Delete dialog are a way of voting about what is wrong with a post, and that they have to select whichever one best fits the situation, even if none of them really fit the situation.
Canned review comments aren't a way of voting and don't affect outcomes in the review system. All that happens when we post a comment that way is that the comment gets posted. Reviewers do not have to select one of those canned comments while voting to delete a post, and posting no comment is, by design, the default.