The XSX isn't running emulation for Xbox One titles.What? It's the other way around. That's why PS5 is limited, because it has the hardware support and is limited by having to run in compatibility mode. While XSX can use the full XSX GPU to emulate and run Xbox One games built for up to 6 TF.
"It likely comes as no surprise to discover that Series X can technically run the entire Xbox One catalogue, but this time it's done with no emulation layer - it's baked in at the hardware level. "
From what Cerny said, both consoles have dedicated hardware support, but XSX's seems to be more seamless.