This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
That sentence raised the same question for me. I interpreted it as not meaning code comments.
However, I think it's a good idea to justify certain design decisions in code comments, for example where a design choice might cause another developer looking at your code to think "why did he do it that way?" or "how does this bit work then?"