This statement is wrong.
The second part of the sentence is actually correct - the one describing what ziggurats are. However, the first part is incorrect - Sumerians were not monotheists. Monotheists believe in one god - for example, Christianity is a monotheistic religion. But Sumerians were polytheists - this means that they believed in many gods, such as Enlil, Enki, Nergal, etc.
