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In the example with natural numbers and lists encoded as W-types, the text suggests that f(12) = 0 is "representing the constructor for zero, which takes no arguments". The constructor for zero is , where is any function (here assumed to be polymorphic). It is difficult to see how f(12) could represent this constructor.
Similarly, the successor function is encoded by , where is defined by .
I propose moving the examples to after the introduction of and updating the examples accordingly.