In wp_list_comments(), what is a short_ping?

This option is used to output a pingback comment. It’s passed to the Walker_Comment class, and if it’s set to true, it will output something similar to this:

<li>
    <div class="comment-body">
        Pingback: <a href="https://wordpress.stackexchange.com/questions/348102/author-url-here" rel="external nofollow" class="url">Author Name</a>
    </div>

The author URL is the field that users enter when filling out the comment form. Note that the closing </li> is missing intentionally.