All agreed. The crew never gets the benefit of hindsight.
On reflection, my point about the captain's decision to turn left to climb out is not a good one. It seems reasonably obvious that he was not, at that time, certain that he was in McMurdo Sound. However, shortly beforehand, after completing two descending orbits that took the aircraft through a hole in the cloud cover, the nav mode was deliberately re-engaged, so that the aircraft would again lock onto the nav track. As Mahon points out, the captain, at that point, must have believed that he was in McMurdo Sound. It is possible that he became slightly less certain after he levelled out at 1500 feet and after the flight engineer said "I don't like this", and then rapidly reassessed the situation after the GPWS sounded. (Some criticism had been made of the slight delay in attempting to climb out after the GPWS sounded, but it should be noted that the first officer had just recommended climbing out to the right, which the captain was starting to have doubts about. The GPWS sounded and then the captain had to decide between the left and the right, which he does in a remarkably short time, and manages to get it right.)
So it's difficult to argue with Mahon's finding that the crew believed they were in McMurdo Sound. If that belief changed, it only changed during the final few seconds, by which time it was too late to recover.
Whether the crew were entitled to believe they were in McMurdo Sound is a different issue.