> sarcasm is simply not a thing in Japan, and people aren't (I'm tempted to say can't be) sarcastic. It doesn't occur to them to be it.

Japanese culture absolutely has sarcasm, it simply manifests in an unfamiliar way. Homegoroshi (killing-compliment) and using a flagrantly inappropriate politeness register are some of the most common forms. Sarcasm, and more broadly humor, are highly contingent on culture and language but they're also cultural universals. They're just usually not that recognizable without familiarity with the culture and strong social skills.