Wow, such a mystery why diners -- which typically exist to serve "comfort food" -- don't serve jellyfish or shirako (sperm sacs of cod). Really glad NYTimes is covering this essential story.
"Diner" here means "Americans who eat food," it's not referring to the type of restaurant also called a "diner." They're just talking about American eating habits here. I'm giving you the benefit of the doubt that you did read it, since you saw "shirako" at the end, but maybe a re-read would be in order.
Diners refers to people who dine not the low end restaurants.
Why do you state that diners are "low-end?" Perhaps not Joel Robuchon but $20+ for a meal, often handmade and/or with unique family recipes, is a luxury for many. Very class-ist comment on your part.