amateur, you are quite correct. The specific wording says:
unless agreed otherwise with the Association for operational reasons. The Association will not unreasonably withhold agreement.
I would expect that in this case they have Association approval.
There is not enough sim space available to train 2 courses a month on the 787 and more crew are still required on the A330 to back-fill what is expected to be significant training movement off the 330 in the new training year.
Forcing Qantas not to employ onto the A330 would have the undesired effect of preventing other crew from getting promoted as there would be no one to fill their previous position and disadvantage new joiners by delaying their employment.
Additionally, the only crew who would be theoretically disadvantaged would be new 787 S/O's who would be more senior to later joining 330 S/O's. However, for the first 18 months in the company Qantas can ignore seniority and allocate a pilot to whatever aircraft they like. So there can be no disadvantaged party for the first 18 months after the 787 starts flying.
This is just my assumption mind you.