From the article: "Ever since Touch ID came out with the iPhone 5S, there has been a tiny coprocessor embedded in the main S-series, and now A-series, processor

From the article: "Ever since Touch ID came out with the iPhone 5S, there has been a tiny coprocessor embedded in the main S-series, and now A-series, processor chip." Uh, the A-series is the "main" series of processors -- the ones in iPhones and iPads. The S-series is just in the Apple Watch.