Prior to this, SiriKit integration for audio apps was not available and was pretty much an exclusive to Apple Music, which we guess makes sense as Apple would want to give its own apps and services an edge over the competition. However, over the years, Apple has slowly opened up Siri to more developers and with iOS 13, it is now available to other audio apps, which Spotify has clearly taken advantage of.
According to the changelog, Spotify notes that Siri will be supported on a variety of devices like the iPhone, iPad, CarPlay, AirPods, and HomePod via AirPlay. However, 9to5Mac notes that during their testing, they found that it did not work. It is possible that we will need to wait for iOS 13.2 before it is enabled, so that’s something to take note off if you were planning on using it with the HomePod.