SMB authentication is generally an oddity and needs to be described separately. I don't see anything on that page talking about how passthrough applies to the different authentication methods so perhaps that should be added. You can't chain NTLM authentication. If it is indeed load balancing between the two domain controllers then your behaviour is exactly what I'd expect.
I think there needs to be a little more investigation and clarification of that page since it may be misleading, unfortunately I don't have the answers off the top of my head so some investigation is required.