If you look at the numbers, you'll see that Metal is about 30% faster than OpenGL on OS X, which is interesting because OpenGL is about 30% slower on OS X than on Windows. Which means they're forcing developers to use their proprietary API to get comparable 3D performance. How is that a good thing?
In contrast Apple exclusively controls the macOS graphics stack top to bottom, so as long as they don't explicitly support Vulkan they're forcing developers to use their proprietary API.
Assuming they knew about Vulkan in time (Vulkan was announced at SigGraph 2014, about a year before Metal's _release_, so that may have been tight), I guess they assumed things wouldn't get better if they replaced OpenGL with another open technology.
Also, from what I read, Vulkan doesn't do anything implicitly, OpenGL does lots of things implicitly. They may have thought that the optimum API lies between the two.
"There is debate over whether Apple make the entire driver set themselves, or just take NVidia/AMD's core code & add their own hardware/OS-specific code to it. The latter seems more plausible to me, but either is just speculation."
I also notice that that page mentions drivers that can be downloaded directly from Nvidia, but gives no indication that either driver is significantly better than the other. That is not inconsistent with them essentially using the same source code.
Saying "Well, but X did $similarthing" as a loose form of justification of whatever Z is doing, like some strange variation of the "not as bad" fallacy. (Though I understand you're point about industry modus operandi)
a cookie if anyone knows the actual fallacy this falls under
1. Apple creates their own implementation of something and pushes the industry forward;
2. Everyone else realizes it's a good idea, and standardizes on something slightly different/incompatible (usually with Apple on the standards committee!);
3. Apple continue using their own thing that Works For Them rather than adopting the standard.
EFI was made by Intel and UEFI was basically EFI 2.0.
Thunderbolt was made by intel (lightning was a cheaper spec from AMD that didn't require expensive active cables). USB-mini breaks and displayport mini is big. What we got instead was USB-C which unifies both of them. Thunderbolt used displayport mini, but the latest 3.0 spec uses USB-C. The fact that all three technologies are starting to unify on the same connector is very beneficial and none of this is dislike for Apple.
Likewise, AMD made the mantle APIs for lower-level work and then turned them over to Kronos where they became the basis of Vulcan. Apple's metal API and doubling down by leaving the Vulcan work group is either "not invented here" or vendor lock-in.
How is Metal any different from DirectX/Direct3D in this regard?
Baby steps :)