They've explicitly said that they are seeking to reduce the surface area for bugs by initially removing cross platform compatibility, and then exposing what primitives the OS needs to supply to make it possible to port. This seems like a much much saner approach to me than trying to work through every quirk in every operating system known to man in the one project itself. Any OS that doesn't implement any of the primitives can legally go and take OpenBSD's implementation and use it if necessary (and hopefully contribute money or code back to them).

I'm not sure why you brought up OpenSSH, since it's by far the most widely used SSh implementation in the world...