This is not an OpenGL library, but Vulkan is another multi-platform graphics API and some references tells that MoltenVK in current state is solid enough for using in real projects, and Vulkan 1.1 is expected to give more features than outdated OpenGL 4.1 (though, I cannot confirm this personally, just my expectations).
In contrast, macOS is much more closed system, where all graphic drivers are part of the system and cannot be (normally) updated without updating system itself. Graphics card vendors provide drivers independently from Microsoft and OpenGL capabilities can be implemented without Microsoft approval.
On Windows, system-provided opengl32.dll doesn't actually implement OpenGL but is rather a proxy-library dynamically loading functions from a driver provided by a graphics card vendor. Built-in OpenGL on macOS works a little bit different from other platforms like Windows or Linux.