Unfortunately, it's a bit of a vague and confusing statement. In good object-oriented design, you express an "is-a" relationship using subclassing. Polymorphism is a mechanism that lets you treat instances of subclasses of a particular class as if they were instances of that class itself. So I think you could say that classes in an "is-a" relationship can, and should, take advantage of polymorphism, and that's perhaps what the K&B statement was getting at.
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