[xxpor]

The original version would be GPL v2 right? If that's the case it'd make sense that the two don't match because you can't reuse the code for FreeBSD. You'd want a completely clean implemention just to avoid any appearance of impropriety, unless the new implementation was done by the copyright holder themselves.

[siebenmann]

That particular file seems to have been taken from Jason A. Donenfeld's original, which was dual-licensed GPLv2 or MIT and so legal to import (under MIT) into FreeBSD. I don't know which upstream version of the file it comes from, but it's definitely very close to the version in https://github.com/WireGuard/wireguard-monolithic-historical .