How is "how to tell a true war story" a metacommentary?

A metacommentary is the term used to describe a narrative which directs the reader's attention to the text's purpose and positioning

"A true war story is never moral. It does not instruct, nor encourage virtue, nor suggest models of proper human behavior, nor restrain men from doing the things men have always done ... As a first rule of thumb, therefore, you can tell a true war story by its absolute and uncompromising allegiance to obscenity and evil."