My intuition based on what I've seen and read is that a lot of the later chapters of the "12 chapter book about compilers" are in draft form, having accumulated over the 50 years since Knuth started writing. As Knuth notes in the fascicle, SAT has proven to be the core of the book (and thus Combinitorial Algorithms). In the beginning, it looked like the hard problems be limited to things relative to memory. It turns out that the hard problems involve pruning spaces larger than the universe.