Slick marketing for sure but it worked for me. I would have ended up upgrading around Christmas anyway but this encouraged me to go ahead and take the plunge. From what I've read, the backwards compatibility in this instance is pure emulation of the 360 operating system. The One operates as if the 360's OS were a game called "Xbox 360" bringing up the prior system's dashboard and functioning as the old console would while allowing the features of the One to run concurrently alongside. Should allow for no change in online play and even cross play between 360 and One owners. Requiring little more than publisher permission to emulate an old game should mean for a lot more titles to be available but, I agree that only time will tell.
Again, that's just what I've read, so who really knows. I still found it to be a solid gesture of good will toward a long entrenched and invested fan base. As well as an FU to Sony's overpriced streaming service.