I think layers is kind of a bad name for what they are really doing... it is just material blending!... their material model uses a unified BRDF lighting equation controlled by a series of blendable parameters usually in the range 0 to 1, this parameters are stored in a texture(s) and processed in a single pass. Since there are parameters controlling if the material is plastic or metal, its roughness and such different materials are achieved.
(with the name layers I would think more of a system where the result of the last material layer is input of the next)

This is interesting but it's not something really groundbreaking. These parameters can be stored in the GBuffer, so that kind of defeats the purpose of this whole system?

What I originally though is that comment you that in your parenthesis :)