Metal is not a new framework

Metal was released on the same year as Swift, could have been a way to show off Swift, instead used Objective-C with Swift bindings.

That’s because the Swift team almost certainly worked independently of the Metal team.

Hardly given that all Objective-C features since Objective-C 2.0 were aimed at improving the Swift interoperability story, as Chris Lattner has mentioned in a couple of interviews.

How is that related to what I said?

They were aware of Swift, and decided to make the upcoming OpenGL replacement framework in Objective-C instead of Swift, and only provide Swift bindings instead of doing it the other way around, implemented in Swift with Objective-C bindings for compatibility with "legacy" code.

Who is "they"? Chris Lattner works on compilers under Developer Tools. The Swift and Objective-C teams share an office and are often the same people. Of course Objective-C is going to get new features to help import it into Swift, because the whole point of Swift was to make a new language that worked well with the old one. Basically nobody outside that group had any need to know of the language at that point, especially since it wasn't ready for system use anyways. I would not be surprised if the first time most of the Metal team even knew Swift existed was when Craig introduced it on stage at WWDC.