Without getting into a philosophical discussion about the seniority of the cadets, the reality is that these cadets who have just joined have seniority numbers from a couple of years back. This means that they're the 'senior' people on course and therefore get allocated to the senior aeroplane. This means the 744.
It'd actually be cheaper for QF to send them to the A330 as a lot of them are already A320 endorsed and so they'd be looking at a differences course rather than the full 744 type course.