In the US you need an IR to fly above FL180, which he did on his round the world trip. (There are glider pilots who have an IR just so they can do this).
The valley he was in is quite wide, I've flown through it and it doesn't seem like a canyon. I measured it on Google Earth, there is plenty of room to turn especially in a Decathlon - I've practised canyon turns in one and it is amazing how little space you need. It's hard to see how he could have flown straight into a mountain like that unless he was in cloud.
It's all very strange.
n5296s