So he didn't know some aspect of polymorphism. Is that pertinent to his book? It's not as if using if/else constructs is bad or wrong. In some fields of development it would be preferred, for blatant clarity.
Does it follow that his book is bad or wrong because it was not written with understanding of this aspect of polymorphism?
I don't rightly understand why you would choose to assume the worst possible interpretation of that line and take it as grounds to heap scorn upon the writer (it strikes me as a tad uncharitable, and needlessly hostile), but
Does it follow that his book is bad or wrong because it was not written with understanding of this aspect of polymorphism?