I actually took that specific course from Professor Siek last semester. (You can find my name on that page.)
Doing the homework was the only way to do decently on the quizes, or the exams. Most of the questions on those tests consisted of "write out the code for the $x portion of the compiler". So if you didn't already understand how that bit worked, you weren't going to figure it out in 10 minutes, and get any of the points.
The low emphasis on the homework allowed him to have basically no restrictions on students working together, which is rather handy for most folks. And it still required you actually develop an understanding of the content.
Doing the homework was the only way to do decently on the quizes, or the exams. Most of the questions on those tests consisted of "write out the code for the $x portion of the compiler". So if you didn't already understand how that bit worked, you weren't going to figure it out in 10 minutes, and get any of the points.
The low emphasis on the homework allowed him to have basically no restrictions on students working together, which is rather handy for most folks. And it still required you actually develop an understanding of the content.