I'm no computer expert or anything, but from what I've read about the xbox one x so far, they're saying that all older xbox games will run smoother, have better framerate and shorter loading times on the new console. Supposedly it will eliminate any jagged egdes on things, have better colours and just generally look slightly more nice than on a standard xbox. To make full use of the extra power the games require extra input from the developer to make it a 'xbox one x enhanced' title. That will involve things like 4k and HDR etc.
I believe that 360 games run on some kind of emulator, so that could be why they dont run any better on more powerful hardware. In fact, I'm pretty sure some games run worse, I remember reading about it somewhere. I could be wrong though.