It may not be based on what the mesh's creator considered repetition, but repetition is encoded within the mesh. Not sure if the mesh builder discovers some of the repetition itself.
I'm not seeing what you claim to be seeing in that demo video. I see a per-triangle debug view, and a per-cluster debug view. None of that is showing repetition.
Not necessarily. Nanite compresses meshes (including in-memory) _very_ heavily, and _also_ streams in only the visible mesh data.
In general, I wouldn't think of Nanite as "one thing". It's a combination of many, many different techniques that add up into some really good technology.
I don't want to estimate storage space right now, but meshes can be stored very efficiently. For example, I think UE uses an optimization where vertex positions are heavily quantized to just a few bits within the meshlet's bounding box. Index buffers can be constructed to share the same vertices across LOD levels. Shading normals can be quantized quite a bit before shading artifacts become noticeable - if you even need them anymore at that triangle density.
If your triangles are at or below the size of a texel, texture values could even be looked up offline and stored in the vertex attributes directly rather than keeping the UV coordinates around, but that may not be a win.
Look at a terrain example:
https://www.youtube.com/watch?v=DKvA7NZRUcg