RAD ALT is the absolute last resort. If that saves your bacon you have already seriously "Fuc?ed up".
The issue with this accident is the fact that even though somebody was possibly "aware" of the danger the problem is the level of severity of the danger.
Glassy calm lakes on perfect days look totally benign and therefore apparently of no consequence yet the fact is the complete opposite. Speak to a floatplane pilot.
Whiteout and sector whiteout is another dangerous condition that also looks totally benign yet is extremely dangerous. Many have come unstuck with the most notable the Air NZ DC10 into Mt Erebus. Flight visibility ~ 300 miles.
We all get told about these "dangerous" flight conditions but nobody allocates a level of just how dangerous.