It's almost certainly referring to pre-OSX versions. However, if it ends up being handled like OpenSSH, the core will be OpenBSD only, with porting handled by another team.
If the cleanup turns out well I assume it'll get ported back to other platforms. It's very hard to change code with a bunch of stuff ifdeffed out (since you're basically guaranteed to break that stuff regardless of how hard you try not to), so even if they cared about other platforms this would probably be a reasonable approach if they're going to make major changes all over the place.
Is this going to be an openbsd only fork (I guess linux will get a build of some sort as well).