AFAICT this works over the same pin that could carry ARC which is arguably more useful to most people and usually available at least on one port. However, devices could also communicate via DDC or CEC too although rather low bandwidth is available it could be enough for some telemetry. Its probably possible to do this via HDMI info frames too somehow... or via direct wireless connections between these devices or infrared or.... the point is there are quite some ways to achieve communication between connected devices already without it.
