Well from that picture - I stared at it for a few seconds, attempting to imagine I was in the pressure situation of hijacking an aircraft, surrounded perhaps by a mass of unfamiliar stuff - I would agree with the Washington Post's comment that switching it off would be far from obvious (note that you felt it necessary to post which part was the 'CDU' and where the off switch was located). A bright big red button on the top would be obvious. Four of these things were turned off in four different locations of the aircraft before any of the cabin crew were able to punch in a 'hijack' code - something they would all presumably have been trained and drilled to do. While maybe not impossible, this is something that seemed and remains highly peculiar.