So why (and how) does QANTAS QF8 use Airbus-380s on that route?
Yes, I now see that QANTAS QF8 uses Airbus-380 from DFW to Brisbane.
Again, previously (prior to 2015?) Qantas only used the 747 and they told me the 380 only had capacity to go LA to Brisbane & Sydney - DFW. So the 380 must have increased their capacity.