From MacRumors:

When it launched, Apple's HomePod smart speaker did not count toward an Apple Music subscription's device streaming limit. This meant that single-user Apple Music subscribers could stream one song on an iOS device and another on HomePod simultaneously, without one stream ending the other. Recently, this ability has disappeared for many Apple Music users, who are now unable to stream music on both HomePod and an iOS device at the same time.

From my HomePod review:

That includes HomePod not counting against any Apple Music device or concurrent stream limit — set it up one or more HomePods with your iPhone or iPad, leave the house with that device, and anyone who stays or comes home can still listen to Apple Music on any or all the HomePods you've set up.

Back to MacRumors:

While single memberships transition to this method of HomePod streaming, Apple Music family memberships will be able to continue to stream multiple songs on both iOS devices and HomePod without any of the streams getting interrupted. Moreover, when HomePod interrupts music streaming on an iPhone, a new pop-up box on iOS displays an option for single membership subscribers to upgrade to an Apple Music family plan. The box explains that up to 5 other people can stream their music at once using the family plan.

So, regardless of what any of us may have previously heard or experienced, this — single stream for single accounts — is the way HomePod and Apple Music were always meant to work together (and how all other Apple Music devices have always worked). If you want multiple simultaneous streams, you can get a multiple (family) account for $5 more a month, which works out to a pretty good deal. (It's what I do.)

Obviously, two HomePods in a stereo pair still counts as one device, but two separate HomePods would count as two separate devices.

I regret if my original reporting and review contributed to any confusion. What I wrote was my understanding and experience at the time. I try desperately hard to get everything right, always, but sometimes things shift or change or are just complex enough or involve people enough that I have to come back and make corrections or updates. And I'll keep updating as and when required, just as always.

(I do think it would be better for AirPlay 2 to allow multiple streams to HomePod and Apple TV within the same, small geofence, but I don't know if music licensing will ever allow for something like that.)