I think the inability to allow Star Wars to be more than one thing at a time is pretty limiting, and the strictness by which people are outlining limits and borders as to what the definition of “Star Wars” is or can be tends be one of the most stubborn roots in a lot of Last Jedi conversations. It seems to be part of a desire to justify a dislike of what happens in the movie by going the extra step towards invalidating the product as not being “really” Star Wars.
The Last Jedi uses postmodernism to reaffirm the mythology, and - nakedly, earnestly - celebrates not just the mythology, but the power and majesty of it in its ending. Was the Force Awakens postmodern when it made Ben Solo/Kylo Ren an on-the-nose stand-in for toxic Star Wars fans? I’d say so. Is the Last Jedi postmodern by essentially putting about 40 years of composite Star Wars fan in the film via Broom Kid (hence my user-name)? Absolutely. Short of fans managing to climb the fandom ladder and get industry jobs that put them on camera, Temiri Blagg is probably the single best chance for a large segment of Star Wars fandom to see themselves AS themselves in a Star Wars film. But I don’t see that as a negative thing, or even necessarily against the “rules” of Star Wars. Star Wars was considered “post-modern” at the time, as has already been pointed out. For a lot of people (myself included) the grasp on the concept is inherently slippery due to the ever-shifting idea of what “modernity” even is depending on when the claim is being made. Modernity in the '60s isn’t the same as it is in 2020.
This precisely the crux of the matter. Mythology and a meta narrative celebrating the power of mythology is not the same thing. Whether it needs to be the same, is a whole other question.