I like to call that a leaky abstraction. The author used "UNIX I/O" as a great example. It perfectly hides the complexity and abstraction is such a way that the programmer never needs to know the internals. It has sealed all the juicy complexity in a watertight container that the user of the abstraction never needs to peak inside of.

The auth example may not be. You may need to do validatePassword(user) for passwordCorrect(user) to be true, which then forces you to open up a hole in the abstraction that is userAuthorized(request) and peak inside. userAuthorized() has leaked out its logic, it has failed as an abstraction. Its a box with 3 walls and no roof that blocks visibility to important logic rather than hides away the complexity.

If you're refering to fopen and friends, that's leaky too. Fopen alone has an append mode which was meant for tapes. And binary mode that was probably useful some day, but hasn't been since idk when. Fsync has its own set of trouble.

Read the fine print.