I don’t think they pushed it as a heavily updated title in terms of engine code. They’re pushing it as an updated title in terms of features which have already existed in the PC version they’re pushing them to their max values, and in quality of assets.
You would need a rewrite of a lot of Unreal Engine to take advantage of all of the next generation features that help push resolution and frame rate. For example one of the areas in where you could push frame rate is with mesh shaders and advanced culling of non-viewable polygons. I don’t think Unreal Engine 4 supports that yet.
You would need an engine rewrite to take advantage of Velocity Architecture features as well.
This is pretty impressive that they’re able to hit 4K a lot of the time at 60 frames per second through just raw horsepower and no engine optimizations. It’s also impressive that VRS is applied without even getting into the game code that much. That’s pretty much means any third-party game will run better on the XSX than on hardware that doesn’t have it, with one line of code.