I'm not doubting this but I've had Comcast internet for years and have never seen this happen. Perhaps because I don't use their DNS? Or because I use ad blockers (but only for the past couple of years). I supposed I might not have noticed if they were replacing ads, but I've never seen an account pop-up.
I believe the ad injection has to do purely with users of the xfinity wifi feature wherein you as a customer get to connect via other comcast customers wifi enabled routers which provide a paid guest network.
