For an intermediate library, it doesn't actually seem to be very much backend-independent. At least the desktop sample [1] has a disappointing amount of code paths with parallel implementations for OpenGL and Vulkan, switched at compile time using "#if USE_OPENGL_BACKEND". I guess this means that this sample will use OpenGL rather than Metal on macOS? They claim there's a Metal backend, but how would one enable that…
Thr shaders are backend specific but rest is mostly generic?