I believe that gender doesn't matter in this chapter because this book illustrates how someone young and vulnerable get mixed up with the War, and never return to their normal self. When Mary Anne arrived in Vietnam, she was the dream girl every soldier wanted to have in their camp but one could see how a young person during War can change in a matter of weeks.
O'Brien lets Rat Kiley tell part of the story because Rat is known for his exaggeration when talking his perspective. A true War story is actually really different from what people think because with all the information told, the public will not be able to understand. Therefore, yes, Rat Kiley's story fits O'Brien's criteria for how to tell a true War novel. Exaggerated stories during conversations are normal as it is proven by psychologists that everybody make up their personal fairy tales.