Apple supports Intel, AMD, and NVidia GPUs. Their graphics pipeline has over the years supported substantially all of the graphics chips produced by those vendors in the past 10-15 years. Their current full feature set may only be supported on GPUs that admit an OpenCL implementation, but that's still every bit as broad as the generic desktop computer GPU market—about two microarchitecture per vendor. Apple's not getting any optimization benefits from a narrow pool of hardware, for GPUs or anything else. The only benefit they get along the lines of narrow hardware support is that they don't have to deal with all the various motherboard firmware bugs from dozens of OEMs.