There is absolutely calling home with HomeKit, at least in most scenarios.
If you're looking to control your home or get notifications outside WiFi/bluetooth range, there has to be a centralized source of truth that you can reach via the internet.
I suppose if one never set up a gateway that would eliminate the need to hit the Apple cloud, but maybe not even then.
What's maddening about what just happened was that it was for just one type of Siri input devices - HomePods. Requests via Siri on iPhones worked just fine. Why is there any distinction at all as to where the request is coming from?
And now I"m getting the dreaded "there was a problem completing your request, try again in a moment message." But at the same time this morning when commands are successfully completed, it's actually, dare I say it, snappy.
So there's some small cause for hope.