It is good advice to create such a unique code, if your application is using documents. It should also be noted that Apple discourages using file type and creator codes. That's why this URL isn't in the book.
It is correct that one may use any file type for documents. Files are supposed to have a creator type, which determines which application is used to read the document. The file type information is solely for that particular application, which uses it to decide how to parse the data. Obviously, it is not a good idea to use very well-known and commonly used file types, such as PICT, JPEG and TEXT, for custom file formats.