A password is a promise someone else keeps

When you log into your bank, you type a password and the bank checks it. Every part of that sentence hides a dependency: the bank holds the list, the bank decides you're you, the bank can lock you out, and if the bank's list leaks, so does your password.

The shared book from the last chapter has no bank in it. There is nobody sitting behind it to check anything.

So it uses something better: not a secret you tell someone, but a secret you prove you have — without ever showing it.