Lucas Matney, TechCrunch:

Music-streaming companies like Spotify will soon be able to let users utilize Siri controls to play music through their apps thanks to Apple’s newly-announced Siri Shortcuts feature in iOS 12.

At a WWDC developer session, the company detailed a new “Play Media” intent it was introducing to developers with Siri Shortcuts that will let users summon audio and video media from third-party apps. The integrations would operate much less seamlessly than controls for Apple Music through Siri, but you would theoretically be able to direct Siri on the iPhone or HomePod to a designated playlist or artist on a service like Spotify, functionality that was previously not possible.

Not clear if this will carry over to allow you to ask HomePod Siri to play a specific Spotify track, but seems likely to me.