This technically is not a "native" read of the sensor. 100% native would be direct authorized access to the sensor. This app and others use access to heart-rate data recorded by the sensor in HealthKit. Devs still needs to create a workout-esque app in order to use the data. It's much different than having a standalone watch app that displays your HR without the need for an iPhone. However I'm sure this API will be made public in the future.