If you're illustrating concepts — such as iterating through an array to find a specific value which will be there because the data structure being examined is defined to contain it at some point, then writing a function that assumes the value won't be there is didactically bizarre.

Zed is suggesting that the teaching point of the code sample be ignored in favor of an altogether different and less useful teaching point. This is not a case of not understanding C (which he cleary does understand) but not understanding how to teach.

At best, his entire point could be addressed in K&R by a cautionary footnote or a later discussion of code hardening.