editedon isn’t the problem here. This line should work:
&tpl =`@INLINE <li><a href="[[++site_url]][[+uri]]">[[++site_url]][[+uri]]</a> - [[+editedon]]</li>`
If you want a nicely formatted date, you can’t use @INLINE. Move the code to a new chunk and delete the strtotime output modifier. (pdoResources and publishedon Date Formatting)
The new chunk should look like this:
<li><a href="[[++site_url]][[+uri]]">[[++site_url]][[+uri]]</a> - [[+editedon:date=`%d %B %Y`]]</li>