Sure, and I agree that the election system in the US undemocratic to the point of being arguably corrupt. But by "rigged", what most people mean is "manipulated in a way that isn't legally allowed". There's no evidence that this happened to any meaningful degree.
If you look at the gerrymandering court cases you'll see that proposed districts get ruled illegal very often. If you think of "the law" as a boundary independent of individual judges then after factoring in human imperfections this shows there must be a number of districts whose layout is against that idea of the law but got through anyways. It's stretching philosophy to say that but arguments on the democratic/legal basis of democracy/law itself are never simple.