The session will provide a "first look" at the kit. You should not expect to support Xbox Live for a while, if the software toolbox is complete by the time GDC starts.
It's still a big move, and reflects Microsoft's shift toward services as part of its gaming strategy. Between its planned game streaming service, Game Pass and its rumored cloud-only Xbox, the company is less interested in where you play and more that you're using its technology. Not that this will serve as a sales pitch for Xbox consoles and Windows, of course. You are gaming on your phone and you use Xbox Live regularly, you might be more likely to pick up Xbox knowing that your contacts (and in some cases, your games) will be there.