The explanation of the URL is that the Single Simple Discussion is a special forum type that is only one discussion thread started by the teacher - it can only ever have replies (not other threads) added to it. Other forum types can have multiple individual discussion threads. The single simple discussion has no 'parent forum' if you like to think of it like that.
The missing export whole discussion button has been reported as a bug - see & vote for https://tracker.moodle.org/browse/MDL-41918. The button appears when you click 'Show parent' too as the .../forum/discuss.php... URL is used for that so that is a work around.
Another workaround is to change the forum type to Standard Forum then alter the permissions to prevent students from creating new threads via the Start new discussions [mod/forum:startdiscussion] capability - effectively having one thread.
I hope this helps