well, I think he was talking about the story of how the fighter generation was known to the public
to my knowledge, everybody started to widely accept the concept of fighter generation from the F-22, and other counties just followed it.
But when Russian got their T-50 out, they claimed it was a fifth-gen fighter, instead of a fourth, because they think their Mig-25 and Mig-27 are superior to their Amecian counterparty.
Later when F-35 was out, Americans just redefined their fighter generation system, and put F-4 in the same generation as the Mig-25/27, so their level count is matched.
China, on the other hand, is still following the vanilla version of the fighter generation system, as they don't have a similar jet to the Mig-25 (J-8 maybe???), nor to the F-4, so to them, J-20 is the fourth generation fighter jet they produced.