I believe GP is referring to the code they HAVEN'T released as OSS. You know... the stuff that makes OSX... OSX and not Unix, and hence copyrightable.
However, in 70+ years, when that code is no longer subject to copyright... I would surely hope they've long switched to newer, more efficient, powerful, and better code... That is to say, if that code would even run on the processors (in whatever way, shape, or form) we'll be using.
As early as we are in our current stage of computing, an additional 70 years may as well be as an eternity. So, I'm not entirely certain that GP's argument is quite valid. I'd be more worried about the code getting leaked from poor patent handling.