From the XFINITY WiFi FAQ[1]:
> Does the new Home Hotspot impact my Internet speeds or data usage?
> The broadband connection to your home will be unaffected by the new feature.
Doesn't sound consistent with the prioritization claim since prioritization still implies the speed may be affected by the guest network. If you're right, then that's kind of a bold (misleading) thing for Comcast to say in their FAQ.
If the author is doing this by using the neighbor's WiFi (as mentioned in some other comments), then I don't see how this has anything to do with XFINITY WiFi specifically, or any loopholes - stealing bandwidth from a neighbor could be done (in theory) with any open/accessible WiFi connection of a neighbor.
[1] https://wifi.comcast.com/faqs.php