I don't think Apple ever publicly stated anything about it, but my $0.02 is on "The latter seems more plausible to me" in http://apple.stackexchange.com/a/213893:

"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.