They are literally the only one who has managed to do it,, and they do so by having a huge layer of compatibility that takes years to update to new versions and support newer generations. We do not know if it would be easier for BSD to support nvidia either ... because they already have nvidia drivers (released by nvidia itself).

On the other hand here we have a random guy that adds support for nvidia gpus, by himself, on a non-linux, non-bsd kernel. And specifically claims he will not do the radeon one because he finds it too much work.

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).