I think the intent of these rules is (and generally has been) to allow Apple to classify APIs into "public" and "private" categories, where the public APIs are documented, and will not change or be removed without plenty of notice, and the private APIs aren't any of those things.

In some cases, private APIs are used as implementation details of public APIs. For example, the symbol "NSNextStepFrame" sounds like it might be part of the implementation of NSWindow. In some other cases, Apple has made early versions of some APIs private before later releasing them publicly -- for example, the Catalyst APIs (UIKit on macOS) was a private API for a while before being publicly released, allowing Apple to test the APIs (and potentially make backwards-incompatible changes) before external developers started using it.