All of the Xbox One games will run on the Xbox Series X they're not on a case by case basis like with 360/OG, the only exception I can see to this would be Kinect based games since I don't think Microsoft will provide support for it, the only ones that are limited are the Xbox 360 and original Xbox games since they are emulated. At least as far as we know Sony isn't tackling previous generations (PS1/PS2/PS3), but I hope that changes somehow.
This is only for OG Xbox and Xbox 360. This won't be the case for Xbox One games, Xbox One games will be available on the Xbox Series X at launch, like I mentioned above. The only exception I could see would be Kinect based games since I don't think they've mentioned anything about it. The case by case for Xbox One games would be only for enhancements.
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Back on topic: Sure the messaging has been a bit messy on Sony's part, but the console hasn't been fully revealed. I'm sure they'll come out and talk about this when they talk about the enhancement their doing to some of the titles. I'm not worried about it.