We have deliberately oversimplified this discussion by assuming that private and public APIs are implemented separately and are not related. While both sorts of APIs must be able to support self-service and must be reliable, there is often very little difference in practice between the way that private and public APIs are designed, created, operated, and used.
This is a good thing because we’ve also found that it is hard to predict how your API will play out. An API launched as a public offering may create the most value when used as a private API (in fact, this is frequently the case). An API launched in private may be used both in private and in public.