Most military history I've read tried to tell the "real historical events" as close to the truth as the author could get.

The things they carried isn't a true history, it's a true story of what it felt like to be in Nam and to Return from Nam.

It's power is in telling the stories soldiers tell, not in finding out if those stories are historically true.
In this manner we learn about the soldiers as they were when they told these stories rather than about a particular historical event.