Headline is ... inaccurate.
It is simply that China's purchasing power has surpassed America's purchase power. Because our stuff is more expensive. Analogy is someone making $100k in the boonies versus someone making $100k in Silicon Valley or San Fran. Neither is technically richer, one can just buy more stuff for the money they have.
US would be the guy making $100k in San Fran.
We have more money (larger economy) but can buy less with it than China. Due to taxes, inflation, high cost of living, etc.