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.