Not necessarily. You can have a tightly integrated system that is not a closed system. There was nothing (technical) preventing Apple from implementing their libraries/framework/graphical frontend on top of a Linux backend.  Feel free to correct me if I'm wrong, but this seems more like a business decision (i.e. 'Not invented here' or "We don't control it") and/or license issue  (i.e. 'GPL is viral') . (I know that Steve was CEO of NeXT and brought that platform with him and that's what they used, but it wasn't <i>necessary</i> for them to do so.)