Here’s the flow. The HomePod listens for a request from a user. If it recognises it as a request meant for a third party app, it sends the necessary data to a nearby iPhone/iPad with the app installed. The iOS device sends the response back to the HomePod, which speaks the reply. It’s similar to how WatchKit 1.0 worked where the connected phone did all of the heavy-lifting for third-party Watch apps.
Requiring an at-home iOS device to handle a third-party app request isn’t much of a limitation, at all. I can’t think of a situation when I would be using the HomePod and not have the iPhone somewhere in the house. There are benefits to a satellite ‘remote control’ approach too: developers don’t have to do anything special to support HomePod, all service configuration will naturally mirror the user’s phone apps, and there’s no need for users to manage another list of installed apps.
HomePod is lacking in capability in key areas, though. The scope of SiriKit is small on iOS and it’s even smaller for the HomePod integration; limited to third-party notes, lists and messaging apps. Some of the SiriKit domains don’t make sense because they require mobility (workouts) or a display (visual codes) but there are others that could be useful if the technical infrastructure could support them. Hailing an Uber from your living room smart speaker is a first-world convenience that HomePod cannot serve.
It would also be nice if Apple opened up new SiriKit domains to coincide with the HomePod launch, to give it more functionality. Third-party podcast and music apps are notable omissions — relevant to HomePod, iPhone and iPad — but there is no news on this front whatsoever.
Most significantly for HomePod is how it behaves as a device shared by multiple people. Or more accurately, how it seemingly ignores any such attempt to be a shared home product at the software level. It seems like one user will sign into the HomePod with Apple ID and iCloud, and all Siri features will be funnelled through that one account. This applies to first-party and third party services.
If you look at the HomePod solely as an Apple Music jukebox, even that has data that is unique to different members in the family: personal playlists and mixes. The first version of the HomePod software appears to have no support for multi-user accounts at all. Not good.