It likely doesn't matter that the URL is relative. It contains a GUID to be unlikely to resemble any real URL, and it's clear enough that they are capable of deep-packet-inspecting all of your web traffic from the way this is already used, so they likely hijack any request to this URL path within their network to capture its contents, and return a 200.
I don't have Comcast so I can't verify, but it would be interesting for somebody to check whether that URL is masked for all Comcast users.
> That's right, this code causes every page served on your system to pop an AJAX request to the wrong URL every 5 seconds, as long as the tabs are open.
I can only hope that they infinitely hang requests to their special URL in the case that user is under the quota so that this is not true. But if it is true, and they are not perfect about masking the URL (edit: it seems like people below on this thread have seen requests to this URL in their server logs), this could be construed as a DDOS attack by Comcast on every owner of an HTTP server via their own customers.