Phil confirms the result of my little experiment:
Another Trackback oddity… Handily you often don’t need to know this URL [the Trackback Ping URL] because Movable Type does magic things (that I’m not even going to attempt to describe) to handle Trackback for you when you link to a Trackback-enabled post on someone’s site. However, if you link to a post on your own site it doesn’t work. But you can use the weird “Trackback URL” to force a link to appear on the old post you wanted to Trackback to. If you got this far you’re brave, or now have a glazed look on your face. […]
Automatically discovering and sending Trackback Pings to your own ‘blog could be quite useful (although there must be a good reason why this doesn’t happen). If I knew a little Perl and had a little time, I’d probably try and patch Movabletype‘s Auto-Discovery routine to do this.