+TSRA
I'm not suggesting that profesionals cannot and do not make mistakes. However I certainly do not think this is an "easy mistake to make". The crew did not misread back a clearance, they did not fundamentally understand what FL two hundred meant. For a pic operating international flights that is worrying. They levelled at 2000' IMC flying towards close in and charted terrain going to above their level. It would be interesting to see how thorough their pre-departure brief was with reference to terrain. It was either covered in the brief and they ignored the implications of the level off or it wasn't covered and they didn't know about the terrain. Either way, that level of operational situational awareness is worrying. They then didn't file a report until contacted by their regulatory authority so either they didn't understand how close they had come to CFIT (again worrying) or they did and thought best not to file.
From all of the above, yes, I would certainly say this went way beyond an "easy mistake to make" and I would question the integrity and professionalism of the whole operation.