Flat light and a few other issues are more prevalent in glacier territory. I've done a decent amount of flying in mountains in variable sunlight and seen things get illuminated before in ways that didn't look "normal", beams of light, certain things illuminated, flat light, "glowing" terrain, etc, but nothing to that extent. It would seem to me that this event is so long ago with so little real data that it's nearly impossible to do anything but guess and speculate, even if you find a scientific explanation that's plausible. Maybe with so much experience and data now we find these things to be fairly obvious in their nature, whereas back in the 1930s with relatively little flying experience and ability to share information, these occurrences seemed much "super natural"?