I don't think the fiber runs all the way to the TV box (someone correct me if I'm wrong). I have FiOS where I live and the way it works is Verizon installed an ONT (Optical Network Terminal) in my basement and then plugged the coax cabling into that box. It would be very expensive to rewire the home with fiber or even ethernet. Presumably if Google was building a CDN they wouldn't want to be reliant on standard wiring in their customers' homes. Also that sleek-looking TV box would probably have to be much larger if it doubled as an ONT (the one in my basement is huge). Seems a little impractical if my assumptions are correct.
Edit: I don't mean to dismiss your idea about the value of Google reserving some space on the disk for their own purposes. Cable & satellite operators already do that today. Technology exists for example that allows the operators to cache household-targeted TV ads on the disk. These deployments are still small scale but I think it's highly likely Google is thinking about such things as a way of monetizing their new network. If you're curious about this topic do a quick search on Google's investment in Invidi and on their partnership with Echostar.
Edit: I don't mean to dismiss your idea about the value of Google reserving some space on the disk for their own purposes. Cable & satellite operators already do that today. Technology exists for example that allows the operators to cache household-targeted TV ads on the disk. These deployments are still small scale but I think it's highly likely Google is thinking about such things as a way of monetizing their new network. If you're curious about this topic do a quick search on Google's investment in Invidi and on their partnership with Echostar.