Strictly speaking, no, it is not nonsense, it is a report of an interview with a man who was John McCain's jailer. Whether one believes it or not depends on a whole range of things (what you think of the man, the story, the circumstances, and so on). But whether you believe it or not, that is up to you, and whether I believe it or not is up to me. One thing that we can agree on is that it is not nonsense, but an account by someone who was there.
John goes on to say that if we doubt ("if we doubt"! as though doubt itself was a bad thing!) John McCain ("doubt John McCain"? how could we?) then we must doubt the inmates at Guantanamo. Well, forgive me, but I do doubt the inmates at Guantanamo, and would be reluctant to make accusations against the US military if their different accounts did not corroborate each other, or if there were not corroborating evidence coming out of the military themselves (such as the translators).
Is it likely that US servicemen were tortured by the Viet Cong, It doesn't seem out of the question. Was John McCain tortured? He says so. The man who held him captive says not. Indeed, he adds that McCain was given medicines and treatment that were not available to ordinary Vietnamese. That strikes me as having the ring of truth.
Certainly his own contemporaneous mea culpa report on Servicemen's Morale in Vietnamese captivity sounds like a long apology for having betrayed his country at the drop of a hat.