While the internal email does indicate that players will be able to play offline, it does not indicate if the games themselves will be tied to a account like Steam games.
We will have to see if Microsoft will allow the used games market to continue, or if it will be killed like many publishers want.
They also fail to mention streaming from a PC to Xbox - that has always required an internet connection for some reason. I used to always stream videos from my PC, and if I ever was disconnected from the internet, I'd get a popup saying "failed to download codec" or something like that. As if for every single movie, even if I've already watched it before, has to download a codec. More likely, they were tracking what you were watching.
We will have to see if Microsoft will allow the used games market to continue, or if it will be killed like many publishers want.