"I've kept quiet a lot of stuff to myself, one of the things the developers have told me is that the demo was crafted with Playstation hardware in mind and that if it had been ported to the Xbox Series X, the Xbox could not run the demo too good because the storage system of the Xbox is different"
This guy sounds like he's often trying to keep a burp down. We can only take his word for this, and don't know who his sources are to judge how much weight they have, but it's inline with what anyone reasonable has been saying here after looking at what Sweeney has said.
A lot of what Sweeney has and hasn't said (including him mentioning the NDAs that still bind) has been interpreted by some as shilling for Sony because they had a technical partnership. I've always seen it as not wanting to downplay Microsoft or anyone, due to the nature of who they deal with and what their Engine is promoted as.
UE5 will scale all the way down to mobile devices. Nanite and Lumen are scalable on PS5, XSX and NVMe equipped PCs. Lumen in the Land of Nanite was very likely something that stretched PS5's capabilities in IO, because that's precisely what EPIC and Sony have been working on together, and that's exactly where Sony has spent their money.
There will be first-party games on both systems that wouldn't be possible on the other. In world detail and traversal for PS5, and in pixel density and perhaps RT in XSX.
Third-party games will depend on each studio, but it's telling that the biggest third-party engine was redesigned around technology developed by Sony, and it's just as easy to scale up in detail as down. As with every PC version of a console game that comes out.
Especially with Nanite whereby you can adjust its various parameters and just use your source material if you want.