I’d rather use the words “traditional” or “typical” when talking about what most people mean when they say something is “authentic”. IMHO a dish can be “authentic” without being traditional if it’s made in the spirit, or according to the “rules”, of a certain cuisine. If a traditionally trained Japanese cook came to my house and made something out of whatever is in my pantry applying the techniques of Japanese cuisine, I would call that authentic.
Of course, that might be hard to apply to American cooking because Americans don’t have the same rapport to tradition as people from certain Asian and European cultures. I don’t think people would get as worked up about someone making a Cheesesteak with emmenthaler as others do when they see someone make a paella with frozen peas and chorizo.
That being said, to me as a Canadian, traditional American food is whatever Americans eat and have been eating for decades. Everything from BBQ to hot dogs and hamburgers to succotash and gumbo to a hot brown or scrapple or crab cakes or whatever your mom made you as a kid. There’s so much awesome food in the US a lot of it comes from immigration, but that doesn’t make it any less American. Just look at what they’ve done with pizza. There are so many regional variations and all of them are “authentic” American.
