"sign up with a new account" I disagree,that is far from clear,what do they mean by new account? Why would you want to create a new account?
I imagine any online service with a user self registration feature has tons of duplicate accounts in their database. People not remembering their credentials and signing up again, etc...
The way I read the sentence above is that you shouldn't do that if you want to use your existing fingerprints which are linked to an existing account.