Hi Geoff the guide does make the assumption that the Pi was connected to a wired Ethernet connection. I personally have not used Wifi with the Pi so I am not sure what kind of problems maybe encountered when using that. But by the sounds of it you seem to be on the right track and you might need to enable the updetach functionality after the WIFI connection has been made. I am sorry I can not be of more help but I do not have the Pi with me at the moment.
You do not want to be deleting any default routes as each internet interface (such as Wifi, ethernet connection 1, ethernet connection 2, etc) needs its own route. ppp0 is used to simulate a point-to-point connection between your Pi and the VPN server but it still needs to share an existing ethernet or wifi interface to gain access to the Internet.