The author describes ctype.h as "kind of a letdown" and as "not sufficient for international text processing" and then proceeds to write an isalpha() that's not portable C because he assumes an ASCII derivative (the code won't work under e.g. an EBCDIC system).

A lot of things in the C Standard Library look odd or pointless until you do some digging and realize that they were often designed to be portable to systems that are either really obscure today, or don't exist anymore.