Unfortunately, the look of the movie is pretty important since it's a visual media. You make good arguments for why the story should be classified as Dieselpunk, but you can make the same arguments about a lot of stories. The setting is what makes them Dieselpunk. The story of King Arthur could be Dieselpunk if it was set in a world inspired visually by the 20's to 50's, but since most tellings of it stick it in the Dark Ages it isn't Dieselpunk. Wizards and Blade Runner can be argued that they are Dieselpunk as well as post-apocalyptic and cyberpunk because of the 20's - 50's visual elements, but they cross genres. You're welcome to your opinions and your definition of Dieselpunk, but I think this is one movie that fails the main lithmus test of Dieselpunk by not having any Dieselpunk visual elements.
All squares are rectangles, but not all rectangles are squares.
The first time I've ever heard of "dieselpunk", years ago, was exactly to define "Mad Max", and even with not much explanation, it got me right away: apocalyptical social nightmare (punk) where fossil fuel (diesel) was key to survive. Just another expression for the much-anticipated derailing of society/total degradation of costumes, back then. The 70's and 80's have movies on this, from Warriors and Escape of New York to Death Wish III. If not by the Third World War, by those damn hippies, I think.
I was a bit surprised when I met this word again, this time associated with a close relation to "steampunk", just moving forward it in some decades. The main difference, I think, it's the total lack of romantism/glamour Mad Max stories have, contrasting to the celebration of the 20's-50's the green side of this train line has. :) But the Piecraftian thing, ok, I got it.
I have that SeaWorld Mad Max, aside The Postman, are cause enough to produce a restraining order preventing Costner approaching science-fiction not less than 300 yards, but that's me.
I just can't accept the meaning of diesel in "Dieselpunk" as simply meaning "gas powered internal combustion." If we go down this path then Dieselpunk will lose any defining character.
I want to stress this because I feel strongly about it. If there is no "decodence," meaning an Diesel Era aesthetic, to the movie then it cannot be Dieselpunk. Mad Max, Waterworld, Escape From New York... none are Dieselpunk.
Both of you made some interesting comments. One of the wonderful aspects of Dieselpunk is indeed that the "punk" does allow for wide degree of individuality. Yet, while at the same time it seems to me that there needs to be something that unifies it and distinguishes it from other genres. If not then Dieselpunk loses its identity.
A good point was made about the majority setting the standard, which I agree. It seems to me that when one looks at majority of the sites and read the articles that uses the term "Dieselpunk" there seems to be one common denominator: "aesthetics / pop culture of the 1920s - 1950s," to quote Dieselpunks Forum Home page. The cars, the music, the art, the fashion... of that magical era is what we post about and would seem to be the place to start since it's a love that we all share.
What leads me on giving an example: Solarpunk.
It is suggested to start a whole new genre on that. Hum... it doesn't seem fit to me, for reasons explained above. I don't think it's just about semanthics, but you can't start a 'genre', but a theme. And, if it gets trendy, then maybe we can discuss.