It's not about who implements an SMB server (it's a separate layered filesystem, smbfs, on Solaris, not ZFS itself) but about the convenience of doing only:

  # zfs set sharesmb=on export/home

and configuring access just by setting (with chmod) the Windows compatible ACLs on the ZFS filesystem itself. Authentication can also work as expected in a Windows shop. Doing all this in Samba is a PITA.

Has anyone tried to use Samba4's Active Directory implementation with Illumos ZFS SMB support (ie, without Samba's SMB implementation) ?

No, why would they? It's not needed, a Solaris server can join a Windows Active Directory ___domain.

I think zdw is asking if Samba can be the Active Directory controller while using the ZFS SMB stack.

Exactly - if it worked, it would be a solid Windows server replacement for storage focused deployments.

win compatible acls are incompatible to posix acls. that's hard to argue away.

ZFS has NT ACLs, not POSIX ACLs.

The file system doesn't implement it, the commands simply let you turn it on or off. CIFS is built into the kernel much like NFS on Solaris/OpenIndiana.

but why should it care about it? it's a whole different stack...