Looks like he answered that same question in the comment section on artstation. "I used these as normal and ao detail blend layers, on top of other blended rock layers. This allowed me to blend in varied low frequency forms at runtime"
So in Bradford Smith's artstation, I saw a section with rock overlays (the last 3 here) https://www.artstation.com/artwork/0owG4 Are these overlays used as a base to generate distinct materials, or are they actually used as seen on that page but to modulate a higher frequency rock material?