Pretty sure they would be UPSHIFTING to blow smoke. You know, since you guys want to sound all smart and such.
Either way, it has nothing to do with Cummins vs Duramax, it's just guys wanting to be superior to someone else. Just the same as you may feel better for not talking shit, they feel better by talking shit. I'm sure there were neanderthals that would brag about their rock hammer having seven strands of leather instead of six. Cat vs Doo, Harley vs Honda, KTM vs Yamaha, BMW vs Benz, Snap-On vs Mac, Red Wing vs Rocky, Jeep vs Toy, Smith and Wesson vs Ruger, Bose vs Denon, Dirt vs Asphalt, Kawasaki vs Sea-Doo, Stand up vs Sea-Couch, Fat vs Skinny, Blond vs Brunette, etc. Everyone does it and wants to justify their choices, and many want to gloat about it.
Just as above there are people dogging on Mopars being pieces of shit, (which they aren't) Others are going to do the same thing the other way.