It's quite possible that they make heavy use of instancing, which might reduce the memory footprint by a lot. Not every pebble or rock needs an individual geometry.
That's why it seems much more plausible to me that they are not storing triangles anywhere, and instead just storing geometry images.
A moment earlier in the video, the poly count of a single statue mesh was given: "more than 33 million triangles."
If you have 1 triangle mesh, and instance it 100 billion times, you don't need to do that.
> "more than 33 million triangles."
The assets the company they mention makes have 100s of millions of triangles, and billions as well. So I wonder if they are not using those original assets after all.
IIUC they are, but they are only rendering at lower triangle counts and the LOD happens automatically.