See the first message of TFA. The author complains that amdgpu has way too many linuxisms that make it impractical to port it to non-Linux ( unless you are already emulating Linux). On the other hand, the nv kernel driver is portable, up to the point that it is easier to port that to Haiku and then change the entire Mesa to use it than it is to port amdgpu (which wouldn't need Mesa changes).
Of course a Linux driver is going to make use of Linuxims. That’s literally the point of a Linux device driver.
Given how performance-sensitive device like a graphics card is, you’d naturally want as few abstractions between your OS and your hardware as possible.
But there is no evidence whatsoever for what you are saying (in fact the nvidia drivers are portable and may perform comparably or even better than the free ones).
On the other hand here we have evidence of how this has made it impossible for any OS other than Linux to provide support for amdgpu (except if you rip a huge chunk of Linux code, which only works if your OS is extremely UNIX-like to begin with), while other OSes support NVidia cards just fine (which do not really have a complete free driver to begin with!).