This is a fantastic resource and a must read but it is quite out of date in many respects. Especially in respect to the introduction of low level APIs, mesh shaders etc etc.
Yes sorry that last paragraph is a bit of a munge it should read:
Rendering it on the other hand would be relatively trivial with a marching cube algorithm AND using one or more rays per pixel [to ray trace against the hull constructed by the simplified marching cube algo - for DXR]
Having thought about it a bit more since (in the other answers) you may be better off with a signed distance field or constructing a specialised BVH yourself as this is a simplified case.
But you've still got to load the chunks from some massive dataset stored somewhere and you can still look/run around in a full sphere at 60fps?
The backdrop pieces are effectively where the rays hit as far as I can tell from your description.
One memory optimisation I can think of is to store a bvh of the empty space and traverse that. Using a signed distance field might also help in the ray marching.
The simple fact that most countries put tight spending regulations on election campaigns should tell you they work and why this level of advertising focus is something we should all be concerned about.