For content like that to be a first-class citizen on iOS/tvOS, the providers (Spotify, etc) would need to expose the relevant metadata, APIs, etc to Apple.
I know that on Apple TV, there were a lot of providers who balked at providing that information, preventing Apple from fully implementing relevantly simple features like system-wide content search.
I certainly don't blame the providers, because they wanted to maintain more branding and autonomy rather than being a faceless Yet Another Source of Content on AppleTV, because at that point they'd be essentially faceless interchangeable.
Seems to me that Apple could pretty easily let any app opt in to Siri integration too. All they'd really have to do is pass commands saying "play song/album/artist named X" to the app, and let the app handle it from there. Add a setting somewhere to let the user choose which music app they want to use for Siri, or make them specify it as part of the command, and off you go.
If Apple offered this and apps like Spotify refused to participate, then maybe we could talk about this going both ways.
Edit: Nevermind, looks like Music apps are explicitly not allowed.
At least Prime video has been announced to be coming to the ATV, although I'm not sure what the hold up is.
So to me Apple Homepod is just another way to try and sell customers into the Apple Music Services Revenue, bound and pulled by the iPhone halo effect.