I think that the point is that you have learned enough so that you
could build the Hack Computer out of Nand gates. It would, howerver, take a far more powerful computer than a typical PC to simulate the more than 5,000,000 Nand gates required to do so.
There is a small missing piece when you get to chapter 3. You are given the DFF as another primitive to use.
D flip-flops can be made out of Nand gates. If you would like to know how, take a look at
this page about Sequential Logic on
play-hookey.com. There's a lot of other good stuff on play-hookey, too.
Alto missing from a complete Hack Computer are the frame buffer (Screen memory) and display and keyboard interfaces.
--Mark