Waren,
There may not be any need to be lower but the pertinent part of the ATSB report is: (my bolding)
The flight crew did not notice that the documented arrival procedure had a lowest descent altitude of 2,500 ft, while the data from the FGMS’s navigational database that was displayed on the MCDU had a lowest descent altitude of 2,000 ft.
So to some degree they were suckered by, for whatever reason, not picking up said discrepancy. There may be no reason for not picking it up, or there may be extenuating circumstances. It's sad that the crew are judged & found guilty before the trial has heard all the evidence.