I mentioned that there are other places in the Book where this information is mentioned, but it's funny how the `?` is described in at least 3 places (your link plus my two other links), but the section you would expect most information about it, called "The ? operator for easier error handling" which is fully dedicated to explaining the operator, does not mention that fact.

My point, above all, is not tied to this specific example, by the way, hope you understand how an informal tutorial about the language does not replace a strict specification - and pointing out that such information is available somewhere if I look hard enough does not disprove anything.