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"because the private key required to create the signature never leaves your smartphone."

Sure. People never get their phones stolen, never buy new phones, people never irreplaceably destroy their phones dropping them in a toilet.

He points this out himself under "Among the problems we have solved to create a practical solution, are:". It is toward the bottom.

He described a local password (basically a passphrase) on the "The user's view of the application" page[1].