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 t

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.