"Partially" is misleading, MoltenVK almost completely supports Vulkan.
At this point the only features not supported are quite minor things, like lack of custom allocator support, and not being able to query pipeline statistics. Geometry shaders are also excluded because they're not implemented in Metal, but they are considered optional in the Vulkan spec.
I think it's more the connotation that 'partial' has. In my head that means that there is still significant portions of features missing. Also given the context, partial implies that it is not useful for its intended purpose. I'd say it's not really synonymous for all intents and purposes.
Exactly. For all intents and purposes, you can now target Apple platforms with Vulkan using MoltenVK with a shared codebase. For most people, the fact that custom allocators are ignored would not constitute incomplete coverage of the Vulkan API.
For what it's worth, rx is already targeting multiple native graphics APIs (Vulkan/Metal/etc.) with a shared codebase through wgpu [0], so MoltenVK isn't necessary here anyway.
It might also be interesting to mention that wgpu is implemented with gfx-hal, which provides a common abstraction for native graphics APIs. But besides wgpu, gfx-hal is used in the implementation of gfx-portability [1], which is a MoltenVK alternative that runs on Metal, DX12, and DX11.
