That's besides the point that monthly caps is a completely nasty rip off. Luckily I never had one. Are they common in Comcast network?
I think what he's trying to say is that as an individual using the xfinitywifi network the traffic counts towards your data cap. As the person hosting the access point, other people using it does not count towards your cap.
I don't think there is such a thing as a comcast account without a data cap.
Note: Enforcement of the 250GB data consumption threshold is currently suspended.
(427GB so far this month...)