The "correctness" the author is asking for here is not what you want from a typical C functions. If you really need this kind of "correctness" then maybe you are using the wrong language and should check our either a high-level tolerant scripting language or a statically typed one.
Kernighan and Ritchie could have decided to write the book as absolute hard-asses, making the most bullet-proof copy routine imaginable, but in the end they would've been writing a different book, not a primer for a language.
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.