Aren't they in essence approximate ray tracing acceleration structures?
No, but having a regular volume of probes lets you index the right ones easily when shading the decoupled surface.
The volume has some more advantages over the surface caching approach: Dynamic or small geometry
can be excluded from expensive RT, and still it's easy to get shading information for them. This way it also allows for volumetric lighting, particles etc., and high temporal lag is acceptable even for fast moving objects.
Downside is inaccuracy. It can not do nice accurate area shadows as seen in Metro, it wastes a lot of perf / storage for empty space if volumetric lighting is not your priority. It also has the same thin walls problem as voxels. (There will be leaking - to prevent it they suggest to make thick walls.)
To improve both, one could use higher probe density at the surface and lower in empty space, but then you deal with traversing SVO or multilevel grids per pixel for shading (like Quantum Break). Doing it all on the surface becomes more attractive at some point.
But i guess you can have a higher update priority close to the surface to avoid wasting performance on empty space.
(hope that's correct... i only skimmed the video for now...)
Ofc you can also combine various tech, e.g. using this probe algo only for 2nd+ bounces. For the first bounce maybe taking a guess form it would help with denoising a more accurate solution like Metro.