No, you're right, what I wrote was idiotic, they clearly are not going to develop the driver code from scratch especially when there's hardware bugs to fix in the drivers. I tried to mean that nobody else built/touched/integrated the drivers, that Apple had pretty complete control over the binary and what did and did not go in it (and likely what kind of code they'd integrate), related to that were things like their long-outdated OpenGL support (2.1 mainline until Lion, which bumped support to... 3.2 — by the Lion release, OpenGL 4.1 was a year old and 4.2 would be released a month later, 3.2 was 2 years old)

Although these days I'm it looks like they write significant portions of the graphic driver subsystems: when released their automatic graphics switching seemed quite markedly different from Optimus and completely independent[0], which is corroborated by them not wanting to restrict their ability to switch back to AMD.

[0] http://arstechnica.com/apple/2010/04/inside-apples-automatic...