"It would be spread over 82,000 square miles, about the size of Kansas, and hold a population larger than a third of the United States. And unlike metro areas that have grown up organically, Jing-Jin-Ji would be a very deliberate creation. Its centerpiece: a huge expansion of high-speed rail to bring the major cities within an hour’s commute of each other."

China seems to have a loose definition of "city". This sounds more like a densely populated area, with multiple cities connected by high-speed rail. Chongqing is another example -- it's billed as one of the world's largest cities with 30M+ people, but it's really a province that China has officially designated as a city.