That's very different. The source code for the userspace portions of the MESA drivers for AMD/Intel are released under a permissive licenses, so OpenBSD (and other BSDs) have been able to modify them to compile under their OS (and get those changes committed to the original tree). With NVIDIA, the userspace portions don't use MESA, so would need some form of translation layer to work on an OS other than Linux.

But, said translation layer would have limited scope; so is a lot more feasible than maintaining a general purpose translation layer indefinitely.