The term "civilization" has a technical meaning in archaeology which includes things such as permanent settlement and a hierarchical social order. Walls and drainage are absolutely treated as evidence of civilization when discussing other ancient civilizations such as the Sumerians and the Maya.
The fact that Great Zimbabwe is not as sophisticated as other civilizations of the same time-period is a moot point because the thing that is interesting about it isn't how it stacks up to contemporary Eurasian society but the fact that it apparently is an entirely home-grown civilization in southern Africa.