It is realistic because, as with war, this flick has short bursts of action with nigh interminable stretches of tedium in between. But it's not. Perhaps it would not seem quite as dull as it does if The Thin Red Line actually had a plot, if there were any sense of purpose to the film. There is none. In that sense it is most unlike war, for in war everything has meaning.
Every engagement has an objective, and survival itself is the unstated purpose in every action.
It might not be fair to call this movie propaganda. Propaganda at least has a purpose, and may have technical precision. This film is a sloppy sentimental mess. It begins with shots of unclad savages living in apparent bliss, and a soldier living in apparent bliss among them. It is an absurdly romanticized portrait of heathen life.