I believe it was the subject of the test who could make the polygraph reading show whatever they wanted, even though it was being administered by an experienced operator.

I think the point is that, since polygraph readings are pseudoscience, it's always the interrogator who picks what they "mean". If this is true, a smart test subject cannot mislead them, since there's nothing to mislead, as the polygraph is just a pressure technique and it means whatever the interrogator needs it to mean.

If the demonstration was performed in some blinded protocol then perhaps there was more room for ambiguity in the results than usual.