Where is that a false claim? Back when Nvidia, 3dfx, ATI and early AMD were beating themselves constantly, there were generations which improved 50+% compared to the previous generation and this happened quiet frequently before all other players died out or have been bought by rival companies. When AMD and Nvidia around the 2000s, pretty much the only 2 major GPU manufacturers worldwide, got together in a backdoor deal where they were trying to no longer get into this fierce battle but rather kinda work hand in hand and inflate the market to make more money, that's where the differences between generations started to stagnate.

AMD being on the loosing end of the CPU market and also getting behind Nvidia in GPU technology ~2010 meant that Nvidia now had the sole crown and they could sell literally anything with very little improvement. That's literally what Intel was doing with their CPUs aswell. So there's no doubt that the generational leaps and gains weren't what they could have been.

Remember why the 1080Ti happened? Because AMD marketing did a darn good thing to sell a product they didn't have and Nvidia came up with a counter to something they never made.

And now, finally, after years, competition is back and as we all can see, the gains between generation(s) so far only the 3000 series compared to the 2000 series are getting higher again