How is that different from a password? PIN stands for Personal Identification Number. Words change meaning all the time, of course, but in this case there’s no reason to call it a PIN when there’s already another word for it.
The important difference is that it is stored on the local device, not the remote server.
Since the things stored on the remote server have been called “passwords” for decades, it seems helpful to call the local thing a “PIN” to help more easily distinguish it.
IMO it’s not more silly than calling a hand-held computer a “phone.”