This was also shortly shown in the keynote: https://www.youtube.com/live/GYkq9Rgoj8E?t=98m33s. It’s not clear to me how much of the background is real 3D, but the dinosaur itself doesn’t require huge resources.However, I did notice that the Ars Technica journalist demo mentioned a 3D dinosaur walking out of a portal into the actual room they were in. They said that it was the most impressive part of the demo and that having the creature as AR in their actual room was way more convincing than seeing it in a VR environment.
In any case, it’s reasonable to assume that Apple won’t quite achieve the performance level of high-end PC GPUs in that small form factor at that resolution, so there will likely be limits to what apps can do in comparison.