It's important to note that in the context of these Apple features, "ID" is for "Identification", not some kind of "User ID" used as a Username.
Username != ID.
> Your face is now your password.
That's a direct quote from the product page.
Re:iPhone X
by jasonkostempski on Tuesday September 12, 2017 @04:52PM
(#15232538)
Yet they behave like a password.
Re:iPhone X
by dkonofalski on Tuesday September 12, 2017 @06:35PM
(#15233600)
No they don't. They behave like a username/password combination. A username is an individual identifier and a password is a confirmation that the identifier is valid for the person it's identifying. TouchID and FaceID confirm both - that it's the correct user that has access and that the user is who they say they are.
Which is why it's called "Touch ID" and "Face ID".