But it's part of Mesa, it's not something you can drop into an app written against OpenGL to translate the calls to Vulkan right?

You absolutely can. It can even build and run on Windows too. I’ve used it to play some modded Minecraft builds where Zink outperformed the native OpenGL2 drivers on my machine. Mainly because the native OpenGL2 driver was terrible at the time for my hardware but it’s 100% a thing you can do.

Some games are even shipping on it [1]

[1] https://www.gamingonlinux.com/2023/02/x-plane-12-now-uses-th...

Where do you think <GL/gl.h> comes from?