True, but I'd certainly consider those "cities" and not "rural" but you can draw the line elsewhere; whatever or wherever you draw the line a substantial group of people live "in a metro area".

The US Census defines 80% of the US population as being urban. As you say, you can draw the line anywhere and I assume the US Census is defining things it as being something along the lines of being somewhat accessible to a city of not completely trivial size. Which is fine as long as people don't read that as 80% of the population living somewhere that looks anything like a sizable city.