How does Smith multiple scattering interact with diffuse subsurface scattering?
The goal of Heitz et al.'s model is pretty much the opposite of subsurface scattering: They only consider surface scattering, i.e. the ray can never enter the material.

Because microfacets are statistical in nature, they can recast their problem in such a way that it can be solved by microflakes, which allows them to compute properties such as the mean free path to derive a heightfield sampling procedure. However, even though they use microflake theory, they still solve the same problem of multiple scattering on microfacets, and their result is still a BSDF, not a subsurface scattering model.

Because it is a BSDF, the path exits at the same location as it hit the surface. This is also mentioned in the introduction:


  Note that our volume-scattering process models the interaction with a surface, but its application in rendering is virtual in that no displacements occur, i.e. the incident and exitant location are the same and the resulting plane-parallel radiometry produces a BSDF.