This is the key paragraph in the article relevant to your point:

  This open-source kernel code is currently split into OS-agnostic and kernel interface layer components. This stems from NVIDIA's proprietary driver on Linux largely being shared code across Windows / Linux / FreeBSD / Solaris. For it to be upstreamed in the Linux kernel it would likely need to be more re-factored to cater to Linux, just as AMD's DAL/DC originally had tough time upstreaming due to its numerous abstractions.