I believe this is only the first point. It's also establishing a hierarchy of these "clumps" (layers), strict APIs representing a single clump for other clumps

I believe this is only the first point. It's also establishing a hierarchy of these "clumps" (layers), strict APIs representing a single clump for other clumps to consume (gateways), and a pattern for clumps to extend functionality of other clumps without polluting their models (extension architecture).