The notes on the site say "4K at up to 120FPS: Requires supported content and display. Use on Xbox Series X as content becomes available".

That certainly implies many games dont support it.

That's only for lighter games. Heavier games still do upscaling: https://www.notebookcheck.net/Cyberpunk-2077-cannot-run-at-4...

I'm not really sure what Microsoft is trying to say on your link but the Series X definitely cannot run everything at native 4K.

And since when marketing material is to be trusted? My understanding is that they do quite a lot of scaling tricks with consoles.