This is addressed in the article: they used many replicated objects. They stored one rock, then rendered it a gajillion times. They claim they were able to do this by applying something like a search algorithm to the objects being rendered.
That's the magic of it -- they are able to create worlds with insane amounts of detail, but in an intelligent way that means they don't need an insane amount of computational power to render it.
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There's supposed to be 42 trillion "voxels" in that demo.
If each "voxel" took only a single bit to store, this works out to require [42 trillion / (1024 * 1024 * 1024 * 8) = 4 889.44352] gigabytes of memory.
How do you store that while you calculate the screen pixels? Modern PCs only have 4 or 8 gigabytes of RAM!
This is addressed in the article: they used many replicated objects. They stored one rock, then rendered it a gajillion times. They claim they were able to do this by applying something like a search algorithm to the objects being rendered.
That's the magic of it -- they are able to create worlds with insane amounts of detail, but in an intelligent way that means they don't need an insane amount of computational power to render it.