They did follow ANZ's descent guidelines though - it was the NZCA's 6,000ft lower limit that seems to be the sticking point, even though ANZ explicitly briefed crews that they could descend to any level suggested or invited to by the Mac Central controllers and that in those circumstances the controllers' directions superseded the NZCA "rule".
In short, what was presented by NZCA as a hard-and-fast rule was diluted to a guideline by ANZ, and ANZ went to some trouble to try to put that fact down the memory hole (via the company shredder).