Is there a good reason for diverging instead of adjusting the
core_sibling mask? On x86 the core_siblings mask is defined by the last
level cache span so they don't have this issue. 

No. core_siblings is defined as the list of cores that have the same
physical_package_id (see the doc of sysfs topology files), and LLC can
be smaller than that.
Example with E5v3 with cluster-on-die (two L3 per package, core_siblings
is twice larger than L3 cpumap):
https://www.open-mpi.org/projects/hwloc/lstopo/images/2XeonE5v3.v1.11.png
On AMD EPYC, you even have up to 8 LLC per package.

Brice