In Indonesia I believe you're allowed to fly up to 12,000 ft unpressurized without anyone on oxygen -- or 14,000 if the crew is on oxygen -- for 30 minutes.
The supposed crash site does appear to be in a direct line between the departure airport at Masamba and Makassar (UPG), the destination, not far from Rantemario peak.
Yet (unconfirmed) reports say the wreckage was found at 7,000 ft altitude. If the crew had intended to take a "shortcut" by overflying the highest mountain range in Celebes, one would think they would have been flying at a higher altitude. As India Four Two pointed out, Rantemario tops out at over 11,000 ft.
Of course it's also possible that they were flying higher, but lost altitude for some reason. Or the crew made a grave error and mistakenly flew direct Makassar.
Since all the passengers are from the Ministry of Transportation, this flight was basically a VIP flight. (Frontier / pioneer flights in Indonesian interior are paid for largely via government subsidies). Hopefully the crew didn't feel pressured to make an unsafe decision.