And you left out several optimized subcases for substrate single slab mode.
Search if(OptimisedLegacyMode == SINGLE_OPTLEGACYMODE_CLEARCOAT) in SubstrateExport.ush, to see these subcases. Though they are named with legacy, but actually they are special optimized modes that just happen to corespond to those separate legacy shading models.
For example, if fuzz is enabled and fuzz-roughness is the same as slab roughness, and sub-surface scattering is disabled, then SINGLE_OPTLEGACYMODE_CLOTH is activated. Code as below
if (BSDF_GETHASFUZZ(BSDF) && SLAB_ROUGHNESS(BSDF) == SLAB_FUZZ_ROUGHNESS(BSDF))
{
OptimisedModeMask |= SingleHasCloth;
}
...
else if ((OptimisedModeMask & SingleHasCloth) == SingleHasCloth && BSDF_GETSSSTYPE(BSDF) == SSS_TYPE_INVALID)
{
OptimisedLegacyMode = SINGLE_OPTLEGACYMODE_CLOTH;
}