"Operating System" is certainly a ambiguous term these days. Two Android-based software systems can differ in user interface, applications, file systems, performance, interaction design and features. Compare Amazon's FireOS with Samsung's Touchwiz with Motorola's Moto X software: Here we have different industry competitors competing against each other. The only special position that iOS has in this scenario is that it is not binary compatible with the others.
Android did certainly not fragment the market. There were thousands of devices with incompatible software systems out there before Android came along. Android is not responsible for all the companies and form sizes and UIs and operators. These were all there before 2008. Android just made it possible for developers to target all these devices with the same codebase.