I see your point Sarcs, but the relevance of those 2 links to current RNP procedures is somewhat outdated.
The equipment has protections by design from the dangers of lone source information as well.
My guess is the simple explanation is that the fellas were too busy gazing out the window, while in severe CAVOK. And in that part of the world, who could blame them!
Find it hard to believe it happened IMC.