It is true that the stock CentOS kernel may be missing some of the kernel modules typically found in the Fedora kernels. However, CentOS offers a "CentOS-Plus" repository where feature complete kernels can be found. The ( dl.atrpms.net/el5-i386/atrpms/stable/
)ATRPMs repository is where you will find additional kernel modules and a complete set of MythTV packages. All of this permits anyone with a 5.x version of CentOS to build a complete MythTV system without the need to compile source code. ATRPMs also has a "bleeding" repository where they hold weekly MythTV SVN builds if you want to try out the latest features (or bugs).
BTW, the guide never claims to be about installing MythTV. It merely puts the building blocks in place such that a user CAN install MythTV (or any number of applications) if they so choose.