Relevant here is the fact that some seem to forget that previous flights to the ice for sight-seeing had made early descents and that other captains had elected not to follow the strict letter of company SOPS, believing they had more than adequate safety margins.
Also relevant, the weather at McMurdo was well below that required for an instrument let down. That was not the case in the previous flights you mention. The only other flight that struck such weather conditions carried out the pre planned sightseeing to the Dry Valley area, and returned to NZ, the pax had been briefed that if weather conditions were not suitable this is what would happen, and in hindsight, and in my opinion, foresight, it should have happened on the flight in question.