I'm not exactly sure, but my understanding is that the XVOD function on the TiVo requires the internet connectivity. I say this because when I first got my Premiere 4 I didn't have any internet connectivity to it setup right away. But once I got the internet connectivity hooked up, sure enough I was able to see the Xfinity On Demand function but I was getting the GSM-11 error.
I understand what you're saying about the regular Comcast provided cable box that you can get Xfinity On Demand feature working without an internet connection. I don't know how exactly that works but if I had to guess it would have to be sending signals via the coax from this box. However, for the TiVo Premiere 4, it's not the coax cable connection for sure.
P.S. One thing I forgot to mention in my other reply is that, in my case, I was also experiencing tuner problems. Some channels tuned some didn't. This was the original Premiere 4 that I bought from Best Buy. Eventually, I took it to Best Buy and exchanged for another Premiere 4. All channels that I subscribed to are tuned and also I had XVOD working. It was hard to believe these new machines can be faulty right out of the box brand new. Bottom line is, what I'm trying to say here is, if it is not too inconvenient for you to consider, exchange your Premiere 4 with another brand new one and see if that resolves the problem.