Noticed in the release notes for the new update of GCM that iOS users no longer have to select a single app to control through the music controls widget as it now controls any and all media apps.
[noparse]https:⁄⁄forums.garmin.com⁄showthread.php?358443-Software-Version-5-20-Now-Available![/noparse]
- Added support for Apple Media Service - users can now control any active music player, e.g. Spotify, as well as volume (requires iOS 8.0 and higher).
You're reading way too much into it. Previously, Garmin watches connected to the Garmin Connect Mobile app running on Apple devices can only control the native Music app in iOS; now, the user can (only) control whichever music player is active. It is not a choice per se, the behaviour cannot be altered in the GCM app's settings, and the user does not specify which app to control – not before, and not now.