The part of the text book we use doesn't look at all like graduate course level stuff to me. I'm sure that once I'm through the basics and at eg. multithreading there will be plenty of challenges for someone at my level, but graduate level challenges? Not a chance.
The full text book is a great big doorstop of a book, so I suppose that things progress all the way from introductory to graduate level, and that's why the preface says what it does.
What do you think of the book? I like it. He writes nice and clearly, and doesn't assume you know absolutely nothing. No spoonfeeding, but low enough explanation level to make the new stuff clear. It's quite fair, for instance, for Unisa to assume that we know a (tiny, tiny, weenie little bit) of C++, the way Jia does; and it's nice when an author just takes that kind of knowledge for granted, and gets on with it, without going into the minutiae in detail.