“Sadly” they are pre-rendered, at least in this case and most of the most popular “this X does not exist” pages out there, as querying a GAN would be very costly if done per page load (requires a beefy GPU to render each image).

In the future we may get a nice balance between more lightweight models and more user computing power that would allow for a different setup, but for GANs that simply isn’t the case yet (at meaningful resolutions).