McCains response to the Swiftboat Vets relating their direct memories about Kerry in Vietnam is telling. He trashed them immediately. Either he knew they were probably right and lied, or he didnt know anything and reacted against them by reflex. The media jumped on his words to defend Kerry of course.
I think McCain reacted reflexively because of his PTSD and a vexing fear that the same sort of revelations could come out about him.
He could have just kept his mouth shut. Or he could have read the book (Unfit...) and researched it himself to give an educated response. But the resonance that he too was Unfit for Command rang his bell.
One family of a MIA in Vietnam complained that he would not help.