The OpenBSD kernel module is also written in C: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/net/if_wg.c...
But the code looks quite different from https://git.zx2c4.com/wireguard-linux/tree/drivers/net/wireg...
That goal may be antithetical to a single codebase supporting multiple (possibly very dissimilar) operating systems.
It's interesting to compare this approach to the approach taken by OpenSSH (and its openssh-portable variant). One has code specific to each OS, the other one has one canonical codebase for one OS + patches addressing compatibility with other OSes. IMHO mostly due to the difference in complexity of each project.