I think what they mean by UV unwrapping is generating a UV map from. Textured model (here the texture is generated by a triplanes network).

It’s interesting for compression purpose, but kinda orthogonal to this method (a good UV unwrapping can be used once the model generated)

Generating an UV map from an untextured mesh. The UV map is stored in vertices as texture coordinates, and together with the topology of the mesh it defines how the texture (images) gets mapped to the mesh. A good UV maps preserve surface area (e.g. every region of the mesh map to regions of the texture proportionally, otherwise you get stretching), have few seams and have little empty space around the texture to reduce size.

There are ways to do this automatically but they're far from perfect. Artists usually take the mesh and literally unwrap it until it's planar, and convert this transformed mesh to the UV mapping. The advantage of this method is that it gives you very good control of seams and texture islands, but it's tricky to preserve surface area.

Those neural rendering methods are very cool because they use light/color fields, but they still have a lot of catching up to do compared to modern 3D graphics.