The ability to match already exists with their iTunes Match functionality, which was extended in their iCloud Music, and their Shazam acquisition would also complement that. That aside, where did Eddy Cue indicate that the EQ analytics are "still" being matched against a centralized database? Perhaps I missed something, from what I've seen / to my understanding, they fed Siri many tracks from their database so that Siri could create a profile of how certain tracks perform in certain rooms and certain placements. But after that seed data Siri should be able to perform the analytics live on data not previously profiled, that's the point of an A.I. seed / learning data set and how A.I. is taught.
It's like feeding an AI various photos of faces so that it can understand the characteristics of a face, but then would be able to identify "a face" not already seen.
In regard to the multi-room support, that's a limited view. Even if Siri is given a "media" domain for Developers to use, I doubt Spotify or other apps will be allowed to be installed on HomePod. Therefore, HomePod will need to interact with the iOS device that has the app. So you are looking at it from the perspective of what Apple potentially does to get it to work, but not what developers would need to do to get it to work. Basically it would require something which is equivalent of AirPlay 2 (or possibly use AirPlay 2 itself), pushed from one device. There's no synchronization or any such thing required, outside of what AirPlay 2 is inherently designed to do.
You're muddling the matter by thinking that AirPlay 2 connects to the internet. AirPlay 2 merely connects to local (wifi) devices, the HomePod then identifies the stream and sees "this is coming from Apple Music on that device, cool, I can play this directly" which then the HomePod plays it directly, but that is not an AirPlay 2 thing. AirPlay 2 is merely a multi-point protocol similar to Bluetooth multipoint. Inherently one device can stream to multiple devices. Downloading from the internet is just separate functionality of HomePod itself, not AirPlay 2. So in the case of iOS device apps playing on a HomePod it would just use AirPlay 2 or something more seamless but based on it, and doesn't require the internet at all (wifi yes, but not the internet).
The building blocks are already there.