Isn't the real issue that the crew descended below LSALT without a positive identification of their position?
They descended because they thought they were over McMurdo Sound based on the aforementioned navigational change and a misidentification of some coastline. However, in the virtual whiteout conditions that prevailed that was a dangerous assumption.
IIRC the SOP prevailing for the flights was that they were not to descend below the LSALT unless they were in visual conditions and had positively identified their position.