- It was explained above, that Apple knew well all along, that there was an effort to make a common low level API. Well before they started Metal. So they purposefully refused to participate.
- C was a proper choice for Vulkan (short of choosing Rust let's say), since it makes it easier to provide bindings from any language than let's say using C++, making bindings to which is way more painful.
- Optimization for hardware happens on low level compilation, so it's a lame excuse not to support Vulkan.
- Faster or not, Apple could observe the result (which was good) once it was ready, and support it. They refused (because Apple).
- Business or not, lock-in is a nasty attitude that's aimed at taxing developers. There is no no need for development tools lock-in, it's a sick anti-competitive methodology, for which Apple is quite infamous.