This has nothing to do with licensing. OpenBSD is a different kernel, so different code. Having written the Linux code, I relicensed any similarities between the two to make it amenable to OpenBSD, and remove any doubt about status.
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.