Interesting that he's using OpenBSD as the teaching system. A more practical book might focus on Linux but I suppose it makes more sense to focus on a BSD if focused on Unix "purity".
In college I had a class called Unix Operating System were we used a book called "Guide to Unix Using Linux". I thought it was odd that the class put so much focus on Unix while we were actually using Linux... why not just put that focus on Linux instead? In the "real world" after college, I only touched Unix once, as Solaris running Oracle 11g. Everything else I use is Linux. In the real corporate world, from my experience as a consultant being involved in many different and diverse environments, Unix is rare and only used when necessary. Linux is everywhere, and always used unless something else is required.
No one takes a "How to use Unix" class to learn how to use OSX and Android. In fact, I've never seen a production database cluster running on a Nexus 5 at any of my clients.
By using OpenBSD you avoid GNU-specific shell/build stuff. I learned on Linux, and then my first job I only used AIX. That is when I got punched hard in the face with "GNU's not UNIX." So I would guess it's probably less about purity and more about portability. Also the manpages on OpenBSD are thorough, consistent and articulate, which make it a pleasure for learning.
This is the first step towards the book. Zed's books are incremental in terms of production and he makes changes often in response to feedback at this phase.
I think he builds them out slowly since he's done so many of these. Taking in feedback on the structure along the way. He really did an excellent job with c.learncodethehardway.org/book/
The book teaches the UNIX way of approaching problems with a set of simple tools that complement each other.
[1] http://cm.bell-labs.com/cm/cs/upe/