It does pay attention to the distinct left and right channels of a stereo track and uses them as part of the sound it creates. I suspect Apple doesn’t refer to it as stereo partly because audiophiles have strong opinions about what qualifies as stereo in terms of separation of speakers, whether they’re two independent speakers and so on. And perhaps because it feels that it’s irrelevant to most listeners, that when they hear HomePod they’ll feel such distinctions are academic.
The last laugh on the matter comes from Mac indie software developer Rogue Amoeba:
You also may have heard about the new AirPlay 2 protocol Apple announced last June. It’s currently delayed, and we’re awaiting more details from Apple. It is worth noting that one of AirPlay 2’s announced features, the ability to play audio to multiple devices in sync, is already possible today using Airfoil.
That is right, you need 3rd party software to listen to your favourite music in stereo on the Homepod on product introduction. Hashtag #applelovesmusic.