For cifs sso (which passtrhu essentially is) will only try the first of the options in the authentication chain that supports cifs authentication. If that fail, it stops. Compared to the web, where it tries each authenticator in turn, and finds one that authenticates you, or fail.
So you would have to set an administrator name for
passthru.authentication.defaultAdministratorUserNames=
and a username from AD that you want to be administrator.
But again, CIFS interface is not meant to do backups of Alfresco. You will miss all versions, alla metadata, …
You can even skip doing backups of Alfresco config, because there is no way you can do a restore of Alfresco repository from what you have backed up. Do a backup of alf_data as per the wiki article I linked to. And of course backup the database as per the wiki.