I believe when you cast something, the Chromecast pulls the data from the wifi it's connected to rather than your phone's data connection. If you were to make a hotspot with your phone for the Chromecast to connect to, that would count as tethering. If you were able to somehow send the stream from your mobile to the chromecast without it being connected to your phone's data connection directly, it's not tethering.
Most devices that you need to stream to like Chromecast, Airplay or DLNA devices require you to be connected to the same WiFi network in order to stream anything. I'm not sure how this can be done via a mobile network without using WiFi.
