The Multiple Scattering Microfacet BSDFs with the Smith Model paper describes a statistical model for replacing the masking-shadowing functions in microfacet BSDFs (which account for paths with more than one surface intersection by setting their contribution to 0) with a distribution which can be path traced and allows the a ray to intersect a microfacet surface several times before exiting.
They do this by modifying a volumetric (microflake) model to behave like a heightfield: to never collide with anything "above" the surface and to always collide with anything "below" the surface.
The result, using wording from their slides, is that "the ray can never go through the Smith volume; the model creates an opaque surface-like interface."
Is this model compatible with traditional diffuse subsurface scattering, where a path can travel a perceptible (macro-scale) distance through a surface before exiting? Or is this simply a specular BSDF with no intention of modeling long paths internal to a surface, to be combined with a diffuse BSDF that adds that component?
(Does the microflake model of volumetric rendering normally separate diffuse and specular, or is "diffuse" simply a path that bounces around so many times that the outgoing direction is uniformly distributed?)