Plus the fact that the crew was cleared later en route to climb to FL370 and reported to Brasilia sector 7 ( the sector in which the change to 360 was planned to take place , ) cruising at FL370.
They were replied to be under radar contact. Which mean in normal ATC operations everywhere in the world to maintain that level unless cleared otherwise.(*)
As they never were, the use of that " Flight plan regulations , incomplete or badly understood pre-departure clearance, or pilot's responsibilities to ask for an amended clearance , is total bulls... to explain why they stayed at 370 all the way.
It looks like someone uses this to hide the ground deficiencies later on .
(*) The only point which is still not clear for me is from which point/time an eventual radio failure can be established , and if so, which radio failure procedures did apply.