That McCain had a slip of the tongue is unremarkable. That he didn’t notice he had a slip of the tongue also is unremarkable—it happens all the time. I think (hope) it also is unremarkable that McCain refers to his audience as prisoners.
What is remarkable is that McCain refers to himself as a prisoner, because that’s what he was thinking, ‘I am a prisoner’. It’s how he sees himself, how he feels. I don’t think it began with his time as a POW, either; I think it goes back to his childhood, when he already had a rage problem.