I think the SIGGRAPH talk you referred to is: "A Deep Dive into Nanite Virtualized Geometry" (https://www.youtube.com/watch?v=eviSykqSUUw)

There's also this short high-level intro (2.5 min) that I thought was decent: "What is virtualized micropolygon geometry? An explainer on Nanite" (https://www.youtube.com/watch?v=-50MJf7hyOw)