Now I am even more confused, its very much possible I dont understand.Microsoft's definition of mixer reality is 'able to do both VR and AR', so I was under the impression that you can just use the VR element with games that support Steam VR. That you cant use VR games for AR is obvious, but I dont see why VR wont work. But yeah, if it doesnt then I get why its important that MS invests in having stuff specifically made for this.
Puzzling!
Hardware has to be supported by the software. SteamVR will have to add integration with the headset for it to work (good bet it will). But not all VR games are on steam or use SteamVR, as such they will have to add support for the headset them selves. Sending the video is easy, but the motion controls have to actually be programmed in (there is a lot of people modding games in order to support OR, or VIVE depending on what is missing).