Well, the gag in which the opening battle is revealed as fake was, as I recall, featured prominently in the (limited) ad campaign.
But even if they didn’t do that, let’s think about it a second. It’s clearly a coming-of-age comedy/drama; the war reenactment stuff is disposed of within the first few minutes and brought up again only intermittently. So if the filmmakers demand that the twist be preserved, the advertisers can do one of two things.
(a) They can keep the twist secret, along with everything else afterward. Commercials sell the film using only what comes before that point, which makes it look like a war movie. Value of that gag is preserved; audience is genuinely surprised (unless they watched the HBO special), but also pissed off, because they expected a war movie when they bought their ticket, and now they’re not going to get it.
(b) They keep the secret by revealing nothing about the movie’s first five minutes. Commercials sell the film exclusively as a coming-of-age comedy/drama; the only hint of the militarism, if one is paying attention, comes in glimpses of the jeep the protagonist drives, plus his penchant for camo garb. So the audience buys their ticket, and the movie starts like a war flick. What the hell? everybody thinks, genuinely confused; am I in the wrong movie? And then the gag happens, and the audience says, Oh, okay, I get it, and settles down. This may work, but it runs the serious risk of being more irritating than amusing. Studios, needless to say, do not like running risks they don’t have to run.
The key here, I think, is that the average moviegoer doesn’t want to feel as if the movie (and/or the moviemakers) believes itself to be superior in any way to the audience. There’s a subtle status relationship at work. The average person buys a ticket with as much anticipation (“boy I hope this is good”) as defiance (“I’m the customer, now amuse me, you bastards, or I’ll get on the internet and write a long and pointless screed about how you should be beaten to death with your own camera equipment”). The ad campaign must cater to this need, and give the audience a position from which to feel superior, and to have a legitimate expectation about the film to come.
Note this applies primarily to the movie’s setup; once you’ve got an audience hooked, you can take them along for the ride. The genius of The Usual Suspects, for example, is that it sets up its noir mystery, asking a bunch of big questions in the first five minutes (who’s on the boat? why does that guy look up and laugh? who’s hiding behind the gear?) along with the promise that those questions will be answered. And then the movie delivers on this promise, answering all of its questions and laying out its explanation. And then it goes past that point, hitting us with the additional twist. We don’t mind that the movie has outsmarted us, because we thought we were right with it until the very end. By contrast, a movie like Basic (starring John Travolta) doesn’t work because it ladles on the twists, one after the other, until the average moviegoer literally has no idea what’s going on and gives up trying to keep up with the story.
Now, also note that I’m carefully referring to the average moviegoer, the vast and undifferentiated mainstream that doesn’t see subtitled movies or pay attention to directors or entertain the thought of doing anything else that might be cinematically challenging. Robert Zemeckis has referred to the commercials for his movies as “selling Big Macs,” because the average filmgoer wants to know exactly what he’s going to get when he sits down in the dark just as much as he wants to know exactly what he’s going to taste when he bites into a McDonald’s product no matter where in the world he orders it. That’s why Zemeckis gave away the triumphant escape in the ads to Cast Away, because he didn’t want people to be scared off by the thought that the movie might end with the guy dying on the island. He wanted the audience to know that things would be okay, that they’d be safe buying a ticket without fear of getting “betrayed” at the end. And it worked; despite the fact that the commercials for Cast Away gave away the whole story, or perhaps indeed because of that fact, the movie was a big hit.
So while in the idealized world of Great Cinema it might make sense to sell a movie with only bare-bones information, in the real world it wouldn’t work at all. Imagine a text-only commercial: “Out of Time is a new film noir set in Florida, starring Denzel Washington. It’s directed by Carl Franklin, who made Devil in a Blue Dress with Denzel and before that One False Move. Starts Friday.” Based on that alone, I’d see the movie (and I fully intend to). But most people wouldn’t.
In the real world, then, you’ve got to give people something to hang on to. And in your specific example, and for movies at large, that means basically everything in the first ten minutes, and indeed more like twenty, is fair game to be revealed. It’s what establishes what the movie is about, which is how most people decide whether or not to see it.