Across the board in Chinese media, all describe J-20 being the 4th gen fighter, and many go in length explaining the generations of fighters. They all claimed the American classified F-22 and F-35 as the 4th generation fighters, and hence, they adopted the American classification. However, in the USAF and almost all reference in the US aerospace industry, F-22 and F-35 are all regarded as the fifth generation. F-15, F-16 and J-10 are forth generation.

It seems like there was one source in Chinese media mistaken the generation information, and everyone else basically refer to that source which got it all wrong.