That is utter nonsense. There were several reasons we "won all the battles but lost the war" in Vietnam and that is not even one of them. Quite the contrary actually.
The moral legitimacy of Vietnam was based on the lost lives of 50,000 Americans and it was very real. The liberal communist socialist democrat cowards and parasites of freedom and their ilk smothered it with nonsense and that is what obscured it. LBJ was a coward and his conduct of the war led the parasites to believe their obscuration was legitimate.
One reason our moral legitimacy in Vietnam was undermined goes back to the early days. JFK, persuaded that it would please Vietnam's Buddhists, ordered the Catholic president of Vietnam, our ally, to be assassinated by the CIA. (Perhaps, too, he was worried that his own Catholicism was a political liability unless he bent over backward to be a bad Catholic.) That was a serious miscalculation, as well as a criminal act.
But at the time we pulled out, the South Vietnamese certainly were still willing to fight. The problem was in the U.S. Not that we "lacked moral legitimacy," but that the fellow travelers in the media persuaded many people (not a majority, even then) that we lacked moral legitimacy.
The whole picture of the war was enormously distorted by Hollywood, as well. Most of us get a kick out of watching "Apocalypse Now," but it's not exactly an accurate picture of allied military operations in Vietnam. The whole war was turned into a hippie caricature by clever propagandists, and the fallout of that deception is with us still. That's just one reason why John F'n Kerry can't be let off the hook.