I am confident that this incident had nothing to do with the terrain. The fact that there were hills just sealed their fate slightly earlier than if this had been flat terrain.
Apart from the fact that if they had been able to descend to sea level, one or both pilots may have regained consciousness (if decompression was indeed the cause) enough to establish level flight or a climb while they came round enough to recover the flight. That of course, is assuming a serviceable aircraft other the pressurisation problems.