The key being that if you've got a technology that works on both server farms for production and workstations for development, you support that so that your OS is a viable candidate for the developer workstation. I don't see a Metal port coming to Linux in any reasonable way any time soon, and I don't see researchers giving up OpenCL or even OpenGL any time soon, so it just means that Apple is going to forego that business.
With the recent github purchase it gives the oddly dissonant experience of having Microsoft being the 'developer friendly' OS company and the MacOS being the 'developer hostile' OS company. Where, and this is important, support for cross platform tools determines hostility or support. I would not argue that Apple is not the best development environment for the Apple platform, or Windows for the Windows platform.