I found a post that does a better job explaining this. I haven't been able to find anyone debunking this. Also it's very much in line with multiple things I've heard and what I just said above. Right now XDK games not trying to tap into RDNA2 are out-performing GDK games. It's from a game dev on Eurogamer:
Native Series X|S games have to be built in an entirely new development environment. As such, the tools aren't quite as mature as the old one. DF had that exactly right (they've spoken to enough of us, tbh). It's also why you'll see some games patching a simple FPS increase (Rocket League, Star Wars Squadrons), because they are using the game running old code, where we have the ability to utilise most of the system's raw power, but none of its new RDNA2 features. You can literally input a fairly simple patch and pretty much guarantee a 2x FPS performance from the One X version at a similar performance level. In some cases, even games that are said to be optimised for X|S (like Ori) are still using their Xbox One code, with some work on top (hence why it runs at a mad 4k/60 on the Series S!)
Yes, there are migration tools to help move the code across, but the reality is that this is very new for most developers, and it's going to take a bit of time to get used to, and MS have some work still to do. The PS5 on the other hand is effectively an evolution of the existing dev environment, so it's much, much more familiar, and much easier to work with at the moment.
To clear up something that should be obvious - the Series X has more power under the hood than the PS5. There's no escaping that. PS5 is a fantastic console, but there is a difference there. It's likely that for many games over the coming few months, you are likely to see a similar disparity between platforms, particularly big AAA releases (though Cyberpunk may be different - they've been working closely with MS), but in time - and not all that far into the future, you'll start to see the pendulum swing the other way.
Now, that gap is not likely to be huge (Think One X/PS4 Pro), but there is a performance delta there, and I have no doubt it'll be evident within a year or so, particularly for any games being built from the ground up on next gen hardware (which we've not seen any of for Xbox just yet).
Hope that puts a few things to bed. I don't expect this to resonate with anyone who wishes to continue the "my box is better than yours" fight, but I thought it was worth sharing.