The loader is a backdoor to load arbitrary code as long as it is disguised as a Vulkan layer (a specialized shared library). It is intended for debugging, but a layer can hook the API to do anything it wants.

Give me one example of a graphics API today on desktop where this isn't possible. Unless you're a console graphics engineer, you're shit out of luck if you really think you can stop client-side injection. Short of a kernel-mode injection like the crap Valorant is pulling, layer hooks and trampolines are literally how those APIs ship (D3D, OpenGL, Vulkan, etc).