I did write about twelve books and I still consider OP's point to be very valid.
The blog entry is great in that it shows how collaborating over the Internet can lead to improvement but it's indeed a bit concerning that someone who didn't know what polymorphism was had been publishing a programming book.
Clay wrote a book about RubyMotion (a Ruby compiler for iOS), not a book that teaches OOP. His knowledge about polymorphism is not so relevant for this book (I think he "knows" what polymorphism is but he was genuinely amazed by the way you can use it for this particular case).
The blog entry is great in that it shows how collaborating over the Internet can lead to improvement but it's indeed a bit concerning that someone who didn't know what polymorphism was had been publishing a programming book.