The word breakfast itself also existed well before America was founded. The author doesn't claim that eating in the morning is a modern invention, rather the concept of breakfast that is marketed to us today. Don't know why the HN headline doesn't stick to the article's.
A Korean breakfast is completely different from an American one.
So American breakfast is relatively modern?
America is relatively modern
Quite. I have older books.
True although I've been having traditional Korean meals (mostly rice and lightly cooked vegetables) in America for decades. Having Korean supermarkets within driving distance is one of the hidden benefits of living in California.