It’s not full voice navigation. There’s a good explanation here:
First, it’s worth noting that Siri on iOS 10 is not a universal solution enabling all apps on the App Store to suddenly integrate into Siri voice queries. The SDK is focused on select categories of application to enable rich voice interactions with those apps. The developer implementation is explored in more detail here, but the consequence for iOS 10 users is that Siri will work with six categories of third-party app: ride booking, messaging, Photo Search, payments, VoIP Calling and Workouts.
The SmartThings mobile app doesn’t fit any of those six categories, so as of now, Apple is not offering an integration that would work with it. Hopefully in the future.