DSCP values also are completely separate from DOCSIS priority on the cable network. With DOCSIS the provider can create service flows with different priorities and bandwidth restrictions. Most providers only use two service flows (one upstream and one downstream) but if Comcast wanted to give some subset of traffic priority on the DOCSIS network (where they are most likely to see congestion) they would put that traffic into a separate service flow with a higher priority. That is how they do their "congestion management" to top users, top users in saturated nodes get their service flow priority temporary and dynamically reduced (via PCMM) so other users traffic gets first shot at open time slots.
In addition, with DOCSIS 3 it is very possible for the MSO to add a dedicated set of RF channels that only select traffic would use, which in this case it would be the Xfinity video streams. I have no idea if that is how Comcast has implemented this but if so how is that any different to Video-On-Demand on a standard STB? With standard VOD on a STB there is a set of RF frequencies set aside for VOD use. If they put Xfinity XBOX IP traffic on a separate RF channel it would be no different than adding an additional RF channel for VOD and would have zero impact on total available internet bandwidth to the cable node.