Doesn't block post requests from bots if mc-value parameter is missing

After I activated the plugin I still got annoying spam comments on my blog. After digging into the code I found that if the mc-value parameter is not contained in the post request there is no validation and the comment can be posted without any barrier.

public function add_comment_with_captcha($comment)
	{
		if(isset($_POST['mc-value']) && (!is_admin() || DOING_AJAX) && ($comment['comment_type'] === '' || $comment['comment_type'] === 'comment'))
		{
                    ...
		}
		else
			return $comment;
	}

Is there any reason for that? Why can’t the else be like

wp_die($this->error_messages['fill']);

instead of returning $comment.

https://ww.wp.xz.cn/plugins/wp-math-captcha/