MS sells an OS for a open platform dummy. Mac is a closed platform which has nothing to do with what the OS supports.
Platform refers to specific computer hardware configuration. Sometimes even an assemblier and hardware confiq but doesn't really have to be releated to the OS running on top. ie linux and windows run on the x86 platform.
Mac uses a closed platform and an open source kernel. I don't see them opening the source for all their code so what's the difference.
Windows runs on an open platform called x86. It's always seemed strange to me that people think a company that forces you to buy only their hardware on top of only using their OS is somehow less of a monoploly?