Let me say at the start that I have not used Apple's HomePod and I appreciate your view (different to Consumer Report for example) that ranks the HomePod above the Sonos Play 1. However to compare the HomePod (£350) with your Google Home Mini (free from your Pixel purchase?) but retailing for £ 29 is hardly a fair comparison, particularly say for music listening.
But I have also read in places such as RFM (and you might not find this an issue yet) that Siri software fragmentation sets it apart from Google Assistant's implementation. Why does this fragmentation issue exist with Siri? Because Apple has chosen to run Siri locally on each device (ie it is a client side SDK) rather than, as Google does, locate the intelligence in the cloud. Siri is now on iPhone, Apple Watch, Apple TV, Mac, iPad and HomePod and each is slightly different and has a different skill set. For example, Siri on say Apple TV has a different skill set than on the iPhone, and the HomePad is incapable of doing some of the most basic things that you can do with Siri on the iPhone. And the third party skills are on a particular device so if you try to perform one such skill on say your iPhone and hope to finish it on say your HomePad (eg, you move location during the implementation) you are unable to do it. Clearly you have not found this to be a problem yet. But philosophically, I would not want such an implementation as it limits my flexibility.
Contrast this approach with Google Assistant, which resides as one version in the cloud, and which allows the user to access it uniformly and seamlessly from any enabled device. In fact GA now operates at such a high benchmark that if two devices are within voice range, it executes the request on the device which is best able to deliver the service.
I could easily see that Apple's approach with Siri could lead to a frustrating experience. If the consensus grows that the HomePod is an excellent speaker but poorer at implementing other digital requests, it will be seen to be competing on hardware alone which will allow hardware competition to catch up---eg Sonos (your view), JBL, Harmon Kardon, etc.
If Apple does not move Siri and Siri SDK to the cloud, I could see this becoming a more significant issue that limits its popularity.
Meanwhile I am awaiting the arrival on our shores of Google Home Max which Consumer Report ranks at the top.