How stupid can one be? Martin Sheen was never in Cambodia either. That was a movie DUMB A%#.
Apocalypse Now was never intended to be a realistic portrayal of Vietnam. It was a 1970s-vintage spin-off of the Joseph Conrad novel Heart of Darkness, set in Vietnam instead of on the Congo River in Africa.
If you read Conrad's novel, you'll understand why an "accurate" portrayal of the Vietnam War would have made the movie utterly meaningless.