Yeah I suggest calling back and trying again and if they can't figure it out asking for global ticketing desk, or the JV support desk as they would likely be able to help.
You probably got one of the many agents who have no idea how to handle a GUC on partner airlines.

I also see Z5 on KE17 for that date. Maybe agent looked at the wrong flight, since the evening flight DL 7833/KE11 is indeed D8 Z0.