Really sharp analysis here! I'm wondering (and I'd need to do some deeper dives into Lewis' theology), but I suspect there might be something going on with how Lewis understands Justification. Regardless, I think you're right that this text makes very clear that only freedom is only natural to "some." And even exceptions to the normal boundaries (with someone like Aravis) are anomalies that seem destined to become Narnians (or Archenlanders).