It seems that Spotify and Apple have parked their particular war and the music streaming service is focusing on taking advantage of the weapons that Apple offers them, and We can now use Siri on our Apple Watch to control Spotify music, both on the Apple Watch and the iPhone.
One of the limitations that Spotify had with respect to Apple Music was being able to use Apple's virtual assistant, Siri, to control playback from its application. This is something that Apple Music, as a native iOS service, enjoys from day one, and one of the advantages that Apple's service has over the competition. A few months ago Apple opened Siri so that it could be used by other music applications, and although it has taken time, Spotify already allows you to use the Apple assistant on the iPhone, iPad and now, Apple Watch.
You will only have to update your Spotify application to the latest version available in the App Store, and without doing anything else you can directly tell Siri on your Apple Watch to start playing Spotify on both the watch and the iPhone. What you will have to tell Siri is that you want to do it on Spotify, because if you don't, by default, it will use Apple Music. You can even ask for the album or artist you want to listen to, but remember, always say "on Spotify" at the end as you can see in the image of the article's header.
For now, and although in some places you can read the opposite, where it doesn't work is on the HomePod. We do not know if it will be possible and Spotify will simply have to update its application, or if Apple does not allow it in any way. Little by little it seems that the advantages of Apple Music compared to Spotify are being shortened, which is good news for users who will be able to choose the service they like the most without losing functionality.