Their stories have to do with the code of behavior of POWs as written in the military rules and regs.
Nothing to do with mere survival but survival for the mission and national security and dying if necessary for the same
During the Vietnam era, the code for the behavior of a POW was to give the captors nothing but name, rank and serial number. Everything else was excluded. There was no elaborate and lengthly code of behavior.
And any information a POW might have had related to a mission or national security would have had a pretty short shelf life of usefulness.