Thanks for your clarification! This was not clear until now.
I suggest that this be added to UCS documentation prominently as it is easy to miss this aspect. Since I now know that UCS can only join as memberserver and not as DC, the documentation on this seems correct, but a causal reader could also conclude otherwise.
May I ask a follow-up question, on why it is not supported, since technically samba4 can be joined into existing AD as DC?