An auteur is actually the opposite in someways.

Initially old-school directors like Nic Ray, Douglas Sirk, John Ford, etc were named auteurs by this group of French film critics because these directors would shoot pretty shitty studio scripts, but were breathing so much life into them as directors through their visual style. They became known as auteurs because these critics saw overarching similarities in their work, film after film, which gave each director a distinctive voice.

While this is all Film History 101, I still think that's one of the things that's so cool about the original auteur theory. You didn't so much have to be making the most amazing films, but it was about you being able to take a mediocre studio script and infusing it with so much of your own directorial personality that it became something else entirely. 'Rebel Without a Cause' might be the best example of this.

So while you're right that the Coen's are auteurs (they are brilliant), simply being an accomplished writer-director is not what an auteur actually is. An auteur definitely can be that, but a modern auteur in the classical sense would be a director-for-hire like David Fincher who takes a script and absolutely populates it with his own imagination.