Your HSI connection is just a channel on a QAM. Your statement "On Demand & CATV set-top boxes never transmitted data over your HSI connection" is just backwards your HSI data is always transmitted over your CATV pipes. The only core differences between the Xbox content and your TV content is the protocol.
CATV & HSI are services that operate on the same coaxial connection. That doesn't mean they are the same thing. You do not need CATV to get HSI or HSI to get CATV. They are independent of each other and Comcast has provisioned their network as such.
