I don't think that quote is accurate. Kel'thuzad's Scourge and the Cult of the Damned were created based on his knowledge of Maldraxxus: they weren't ACTUALLY from there. Hence why the constructs look different, the ziggurats are constructed differently, etc.
Also, Venthyr are not actually vampiric in any way aside from their aesthetic. There is no use of blood or blood magic. Nathrezim however...