You are right in that a password is only secure if it is obscure. But the “obsure” part of “security through obscurity” refers to obscurity of the system. With passwords, the system is completely open — you know the exact method that is used to unlock it, but the key, which is not part of the system, is the unknown.
If we were to generalize, then yes, all security is by means of obscurity. However, the phrase “security through obscurity” does not refer to this.