It's only a rampant layering violation if you mandate the use of external layers like Linux device mapper as the only allowed way... Or you haven't actually read through the code and assume based on external user interface.

No, ZFS is not "monolithic".

It's just that on the outside you have a well integrated user interface that does not expose you to SPA (block layer), ZIO (IO layer, that one is a bit intersectional but still a component others call), DMU (object storage), and finally ZVOL (block storage emulated over DMU) and ZPL (POSIX-compatible filesystem on top of DMU) or Lustre-ZFS (Lustre metadata and object stores implented on top of DMU). There are also a few utility components that are effectively libraries (AVL trees, key-value data serialization library, etc)

In the Linux world you need to be hard to use in order to prove how pure you are. Anything that is actually easy to use is always considered unpure and bad.