They sort of do have it. You can be playing music on your phone and "tap it" onto a HomePod.
It also works the other way, I can ask Siri on the HomePod to play something, and the details will show on my iPhone & iPad.
I don't know if there is a way to go from iPhone to computer.
EDIT:
That may not qualify as a "handoff" though.
But it does handoff from current device to AirPods when you put them in, so there's that.
I was going to stop by to post about the feature, documented on the above page, under “Play audio on a HomePod using Control Center”
If it showed up anywhere in future, it would be there, I think, in the list of AirPlay devices.
I can say I’ve had the opposite problem, where I’m on Spotify on one device and, while playing, would want to use a second device to preview tracks. But the Spotify model only lets you play in one location at a time. I do like the Spotify model most of the time, though.
Everything about the HomePods seem to be. I once asked what time it was and due to wifi problems it gave me some generic error message. It baffled me that this thing has to phone home for the time.
It's much better on the HomePod Mini, and I think it must have to do with the U1 chip that can detect proximity. My OG Homepod is frustrating to try to use Handoff on.
But it does handoff from current device to AirPods when you put them in, so there's that.