Well, I gave you a way in which it is possible: By reengineering the motherboard to fit an additional controller.
But that would require significant effort across more than just the board and this is a quiet refresh to update the internals to the most modern chipset and nothing more - that's what I was implying. Since the update to Skylake didn't bring that natively, Apple themselves would have to go out of their way to add it in - and why would they do that when the next iteration will have support native?
It wasn't included, but it wasn't missing. If Skylake supported it natively and Apple didn't make the appropriate move, then yes, it'd be missing.