The question is what is a myth and do we believe it or do we think it is a lie. What I get from postmodern is that under that philosophy the myth is a lie. Postmodernism sets out to tear down myths.
What we have in TLJ ends in the opposite. Luke acts, not to create a false myth to tear down the Jedi, but to create a new myth to build up the Jedi. What he does is of mythic proportions. He does some Jedi fakery, but the First Order witnesses his actions and has no explanation. Luke stands on the battlefield of Crait and is blasted by the arrayed First Order forces and comes out unscathed. Kylo Ren takes the field to face him and after his saber appears to pass through him, Luke fades. While Luke’s actions in the OT happened in confined quarters with no witnesses, his actions here happen in the open in front of many witnesses on both sides with few understanding the reality of what just happened. Luke did not take this action to create a false myth that would destroy the Jedi (what postmodernism would do), but to create a new myth to help rebuild the Jedi. And for those of us who know what he really did (Kylo, Rey, and Leia probably understand as does the audience), his actions are even more mythic. Definitely not a postmodern attempt to create a false narrative to tear down the old ways.
TLJ is actually a critique of postmodernism because both Luke and Kylo start out the movie with a postmodernist desire to see the old fall to make way for the new. Kylo is a student of the dark side so his views are never the ones we need to pay attention to. Star Wars always makes it clear that the views held by the Dark Side are not the right ones. Luke is jaded but in the end abandons his darker views and returns to the original path of rebuilding the Jedi. He does, in his own unique way, what he had previously told Rey would not happen - he faces down the entire First Order with only his lasersword. He embraces the myth of the Jedi and takes action to carry it forward. Far from tearing down or deconstructing the old myths, he actively encourages and renews the old myths.
Again, while the views of grumpy Luke and dark side Kylo Ren may seem to be postmodern, the way the story plays out see the end of grumpy Luke and the rise of Mentor Luke and a return to old Star Wars and modern myth like it has always been. Taking grumpy Luke and Kylo Ren as the speakers for the film is a false view that is contradicted by not only the end of the film, but by all of Star Wars lore to date. You never listen to those on the Dark Side, even if they are telling the truth, they are doing it for their own nefarious ends. The Myth of Star Wars is intact after TLJ.