HomePod is not a mono speaker. Apple specifically states that audio is split into left/center/right channels. The issue regarding the "stereo pair" is that two Homepods provide two woofers, so technically every level of audio (high/mid/low) can be split into more than one channel. With a single HomePod, high/mid range is split, while the low can't be split due to only one woofer.
And that's fine because even with multiple speaker setups, low frequencies are typically the same across all channels as low sounds aren't generally directional. We're far better at distinguishing the direction of higher pitched sounds. This is why a 7.1 home stereo will still just have a single subwoofer.
Most fail to understand how amazing what the HomePod offers really is. Until the HomePod, you'd be looking at paying $20,000 to get a speaker with beam forming. They can create a stereo-like experience by sending different tones and frequencies in different directions, so they appear to be coming from different areas of the room as they would with a true stereo setup. This has been noted in multiple hands-on reviews with the HomePod thus far.