I have looked everywhere and they are all pointing back to the 1.0.3 plugin fixing this issue, so I assumed version 1.0.4 fixed it too, but I don't think so...
Error when destroyingan element: [LDAP: error code 66 - 00002015: UpdErr: DSID-031A12A5, problem 6003 (CANT_ON_NON_LEAF), data 0
This is quite old thread but still worth answering:
You should try with latest AD plug-in, there is a new workflow "Destroy a computer and delete its subtree" that is taking AD:ComputerAD as argument but is using a generic action that can take any AD object.
You can take the plug-in from here: Technical preview version of VMware vCenter Orchestrator Plug-In for Microsoft Active Directory and it is already part of vRO 7.0.1 release.