I don't know why the document pointed it out... but I guess it was done for completeness.
One. Access modifiers are for everything within the top level class. So, private mean private to everything within the top level class. Private fields and methods, no matter where they are, either as direct members of the top level class, or any nested/inner class, can be accessed by any code, either as part of the top level class, or any nested/inner class.
Two. In order to inherit from a superclass, you need access to it. So, if you want to inherit from a nested / inner class, then you need it to be accessible -- ie. public or protected.
And finally, three, the key
word in the sentence, is "indirect". Any private member can be assess via non-private setters and getters. So, the subclass can access the superclass private fields only indirectly.
... put these three points together, you get that explanation.
Hope this helps,
Henry