There is a difference between uniformed soldiers who represent a nation and battlefield combatants who are hired killers for terrorist organizations The Geneva Convention rules defining "soldiers", "partisans", etc (and thereby defining those who fight outside any of the above catagories as an "illegal combatant" are rather more extensive than that). The fact that al-Qaeda thugs wear no uniform is one of their violations, but far from the only one.
Under international law, the perps of the Malmedy Massacre were entitled to no process of law beyond "yep, we've proven beyond reasonable doubt that they're SS, all right -- hang 'em high". The US went well beyond that, and McCarthy's defense of the Malmedy perps thus has no rule-of-law justification.
I havn't researched what exactly happened Malmedy Massacre and how McCarthy was "defending" the Nazi's, so I can't comment either way. But I can't see why this should discredit Ann Coulter's book because she didn't include this episode in McCarthy's life. Her book is titled "Treason" and it is about "Liberal Treachery, From the Cold War to the War on Terrorism" not a biography of Joe McCarthy. I think Rabinowitz is nit picking just a bit, It would be like discrediting a book on the Cuban Missile Crisis because the Author didn't include in the story that JFK had an affair with an intern.