From the video it sounded to me that the 100s billions of triangles were from single instance geometry. The models they mention in the video are in the ball park of >1 billion triangles each (that's 12 Gb per model). So if they have 100 models of that resolution, that's 12 Tbs, and whether they instance each model 1 or 100 times doesn't matter much because the problem of having to fit those 12 Tb in triangles somewhere still remains.

That's why it seems much more plausible to me that they are not storing triangles anywhere, and instead just storing geometry images.