The assets for that demo are all lumped into a "Limestone Quarry" collection on their site. I imagine most of it was gathered from one real world location. The other assets (structures, statues, etc), were probably hand-made by the artists, with the location in mind so the visual look would match. Those other assets are not available for download, obviously... there's not enough there to completely recreate the demo, although creating a cave or quarry that looked the same would be easy enough.
While the stuff available on Quixel's site is impressive, I agree that artists will want more freedom to match the artistic vision of whatever they're making. That in no way means that they can't be of the same fidelity and realistic quality, of course. I think the demo was a good idea of what kind of overall graphical fidelity we'll see next-gen, even if we never see those specific rocks ever again.
Also, Quixel's stuff isn't free. There's a free section with some demo models that I've messed with, but most of it is behind a paywall. For a subscription, you get a number of "points" you can spend to purchase specific assets across the site. I've seen the same system at a few other 3D asset places before. As a hobbyist, the pricing isn't bad for me, but it does get up there for larger multi-seat or multi-site licenses for bigger companies.
Depending on the specific object, you still have to be careful how it's used. I was messing with a larger scale mesh of a rocky cliffside, and it looks amazing from a mid-level distance or higher, but it still doesn't hold up when the camera's really close. They're 8K textures, yes, but they're spread out over a very wide area, so a single rock doesn't have a whole lot of fine detail on it. Smaller objects will of course look better up close because that's what they're made for. I'm sure the close zooms they did with the free camera in the UE5 demo were specially done in areas that could stand up to that kind of scrutiny, while other areas wouldn't.
