This is a misunderstanding. To give an example. A perfect 1.e4 player needs a response to every possible move including 1.. f6. But he doesn't need to know how

This is a misunderstanding. To give an example. A perfect 1.e4 player needs a response to every possible move including 1.. f6. But he doesn't need to know how to respond to 1. d4 d5 because he avoids ever ending up in that position.