In: Psychology
Post modernism is defined by stating that all categories are social structures. “Gender fluid” is post modern concept. It’s the idea that calling someone a man or a woman is merely a social convention. And that people often have traits of either category and can either choose their gender or a mix of both
in fight club Jack redefined his entire life and becomes Tyler Durden. At some points in the movie he chooses to be Jack. At other points he chooses to be Tyler. That fluidity is a post modern concept.
Another way of thinking is
Postmodernism is all about breaking down traditional structures of narrative and form in the art world, challenging expectations.
Fight Club has an unreliable narrator, scenes deliberatly showing hallucinations as real memories until revealed towards the end of the movie and numerous weird camera angles and scene transitions. On top of that the plot is about a young man trying to define himself in a world that doesn’t make sense to him.