Your story seems true to me and I find no fault in your approach. And yes any experienced interviewer could have made a more reliable call, but it is less like a coin toss, which is at least fair, it is more likely a predetermined outcome. They could never say your age is a factor, but wink wink, and I am not saying it happened here, sometimes as examiners we just know these things.
I am surprised that considering your background, you are just now discovering what preemployment polygraphs really are which is illegal in private employment since 1988. The Employee Polygraph Protection Act.
Oh yeah, the feds exempted themselves.