This seems a little too sensational to be trustworthy. I’d take it with a grain of salt. China, like the US, is an economic superpower. Economies have ups and downs, and China still has a long road ahead.
As @sik0fewl mentioned. China has a population problem, a huge population problem.
Most of the developed world does but most also have large immigration numbers to offset it. The USA is the same, immigration is the main driver of its population growth and population growth is the main driver of maintaining and expanding a countries GDP.
They scale that china's population will crash over the next 30 years is hard to grasp.
The number of people they have who are of working age will drop by ~100 million. While at the same time the USA will grow in that range.