I don't know about the MoltenVK branding specifically, but Khronos does plan a "Universally portable subset" of Vulkan with tools to target both Metal and D3D12 from one rendering codebase and common SPIR-V shaders.
The project is call Vulkan Portability Initiative:
If I understood correctly, gfx-rs aims to provide an implementation for that initiative. Khronos only announced it in concept, they didn't implement it.