No, I think what he was saying is that the test was commissioned by the defence, and so it would have been up to them whether or not they disclosed the results. I agree that had he have failed the test he would have had rely on Mullins keeping his trap shut, but I assume there would be some sort of confidentiality agreement attached.
I take the view that anybody who scoffs at polygraphs should check with them self whether or not they them self would take such a test, regarding an untruth in their own life. Now I could be wrong but I suspect that if a person did not want to expose them self to the risk of that untruth becoming known via potential deception indicated result, they will back out of the idea of taking a test in the first place.
What is being suggested here is two things. Firstly Bamber could bury a result that wasn't in his favour. Fact is, his supporters would leave him in droves upon there being any sign of concealment, regarding a result not in his favour. Secondly, he is mentally abnormal and therefore either wrongly believes in his own innocence or is able to switch off his reactions, thereby duping the tester and / or equipment. On the second one, I'm not sure. My personal opinion is it's unlikely. Obviously others will disagree.