Well, it never claims to teach you all of the language; it tries to show off some neat parts. Articles like that often don't go into the details of I/O. Especia

Well, it never claims to teach you all of the language; it tries to show off some neat parts. Articles like that often don't go into the details of I/O. Especially articles about languages with REPLs, which have much less need for that since they allow you to interact with your program more directly. (But it's true that many other Prolog resources also ignore I/O.)