Sorry for the delay in answering, alerts on this site don't seem to be firing (!), I've made a note to start checking in more often.
The redundancy is an error. Neither DNS nor AD should be installed after MOSS is installed. This is how it happened:
The steps on page 49 describe an alternative method of configuring AD and DNS. That section (like the one on remote debugging after it) remain from an earlier version of the chapter where each scenario (WSS and MOSS) was described from start to finish. I didn't like the redundancy of that approach, so the chapter was changed to a more comprehensive "no matter what you do next, start with the core Windows Server Basics, then add steps for each major choice - e.g. WSS 3.0 vs. MOSS - as the chapter progresses. The redundant (p. 49) section on AD and DNS configuration was moved rather than deleted.
Hope this helps!