OK, my fault - I totally missed the "how to display active threads on forum home" link in the first post. I can understand your confusion.
It looks like what's going on is the thread info is being loaded in to the page as javascript. So you can't use any php variable replacement in the templates to add info (because the thread variables aren't set), you have to just use whatever is passed in the javascript. I don't see it documented anywhere (big surprise, really), so I just got the javascript and looked at it. It looks like you have 5 things you can use: threadid, title, poster, threaddate, threadtime. And you would use them like you see them being used there, threads[x].threaddate for instance.
You could of course get more info by getting it from the database, but then there is probably no point in using the external data provider thing.
ETA: actually, I thought of something else you could do - change external.php. If you search for
Code:
if ($vbulletin->GPC['type'] == 'JS')
{ // javascript output
you will see how it constructs the javascript. There it has the $thread variable available, so you could add any fields you wanted in addition to the 5 that are already there.