It’s been a long complaint by music app developers, notably Spotify, that Apple doesn’t allow third-party music apps to work with Siri. Before now, it only worked with the official Music application.
It comes after Spotify and Apple were involved in a dispute about expanding the SiriKit feature to App Store developers. It seems that this has now been resolved, and available for anyone to add to their app. Pandora Music has already released an update adding this.
Users of the app will be able to ask Siri to control music, play a specific album, playlist or song and more. It’s not known when the feature will make it out of beta development, but it will likely be soon.