If they didn't use their full force it is by definition not maximal. But that's skirting the point a bit. The quoted passage (and the book) are a very biased take on history in general. Zinn wants the narrative to be that powerful technocrats couldn't overcome the people backed by some mythical cause (communism) whose good is the ultimate destiny of mankind. The reality is Vietnam was in civil war, and super powers were backing differnt sides. The US literally could not use its maximal force because it risked escalating into a larger conflict (China, Russia, etc). The real story is nuanced, complex. The US did lose -- nobody is contesting that here. But the quoted passage (and much of the book) is as much an attempt to spin the narrative as is "The US could have won if it wanted to".