No doubt Becker believes what he is saying, but....it is just impossible to know.
Had Nadal been born 15 years earlier, how many GS would he have won? 5? 10? 3? 6? It is just impossible to know.
And exactly the same with any other player. How many GS would Federer have won had he been born 10 or 20 years earlier? 5? 20? 3? 10?
It is just impossible to know.
All these across-different-eras hypotheticals are totally senseless.
It is more, even inside a period of time where conditions (courts and balls and racquets and strings) stayed more or less the same (for example, from 1983 to 1995 ), it doesn't make sense to say that Sampras was "greater" than Lendl, or Agassi was "greater" than McEnroe, because 10 years apart is TOO MUCH (they really developed in different "eras", played mostly against different fields of players, even if they overlap a bit).
Let alone trying to compare players from that era with players like Laver, Rosewall, Newcombe....from the wood era, or with players from the current homogenization-slow-polystring era.
It is like trying to compare Viktor Barna, Richard Bergmann,.....,Guo Yue Hua,Zhuang Zedong, Kong Ling Hui, Chuang Tse-Tung, Wang Liqin, Jan Ove Waldner, Liu Gouliang,...
It is just impossible.