Binary compatibility is for userspace programs, not kernel modules.
Edit: I'm not having any FreeBSD systems in use right now, so i don't know if they changed the defaults. What i remember is at least the last two versions were compiled into GENERIC, so you didn't even had to compile your own kernel when that range backwards was enough.
Similar thing in NetBSD, a loooong time ago, even their own driver for 3Com905 relied on the equivalent of that functionality. That caught me by surprise, because i used NetBSD like Gentoo, found some Compat foobarjurassicBSD, wondered why i should use that at all, and had no working net. Fun! :-)
I believe something similar is done in Linux. The difference in experience may come from FreeBSD maintaining a stable KBI over a release version (e.g., stable/12), while Linux aggressively does not maintain any KBI stability; hence the need for DKMS/akmods.
This is unrelated to COMPAT_FREEBSD.
(All Nvidia would have to do is build another binary; it's still chatting with the same driver blob internally so it's not really clear to me why they don't bother compiling it for FreeBSD.)