That is not a "notion". It is a fact.
It is not a fact. It is an excuse that happened to emerge exclusively from the Federer fanbase due to inherent interests (to demerit Nadovic's longevity).
There is indeed ATG talent in the current men's tennis tour. Dominic Thiem has ATG talent. It is safe to assert that Thiem had the potential to be a 6-times Slam winner without Nadal and Djokovic (RG 2017, RG 2018, RG 2019, USO 2018 and AO 2020).
Nadal and Djokovic suceeded in stopping the Next Gen, unlike Federer. The ability to stop the Next Gen is a sign of greatness. In every single sport, the ability to stop the Next Gen is crucial in a GOAT resume.
To draw an analogy with the beatiful sport of chess, Kasparov not only defeated his older rivals (Karpov), he also succeeded in stopping the Next Gen (Anand, who is younger than Kasparov), and that is one of the reasons why Kasparov is praised by many as the chess GOAT.
In basketball, LeBron James was the NBA finals MVP this year aged 35. He stopped the Next Gen.
In football, Cristiano Ronaldo and Messi have both won the Ballon d'Or aged 33, stopping the Next Gen in that sense.
Nadal and Djokovic stopped the Grand Slam champion Thiem at 7 Slams: RG 2014, RG 2016, RG 2017, RG 2018, RG 2019, USO 2018 and AO 2020. A 33 years old Nadal stopped a 23 years old Medvedev playing at the height of his powers in the US Open 2019 final. That is to say, Nadal defeated a 10 years younger player in a Slam final. Federer has never defeated a 10 years younger player in a Slam final.
In sum: Federer's inability to stop the Next Gen costed him the GOAT title.