I simply can't believe that's the reason. It's an error message, not a warning. It doesn't check if new comments have been posted to that thread, it doesn't prompt you with the new comments, and ask if you want to continue with the post, and it doesn't make it easy to go back to the original thread to check if there are new comments.

Asking users to type in an external text editor, and then copy/paste isn't a reasonable solution for a message board.