First, no one (who hasn't signed an NDA) knows for certain how it will sound because the HomePod isn't released but there is more to good sound, especially in a casual home environment, than stereo vs mono. Much more. In fact, for most people mono vs stereo is largely moot because for stereo separation to work you have to have your speakers placed properly and be sitting in the correct listening spot—and that's not even getting into room acoustics like treating primary and secondary reflection points. The key to the 7-tweeter array isn't the number of tweeters per se (you're right, on their own a million wouldn't matter) but the processing behind them. Apple promises to use beam forming to to alter the speaker output to counter the size/shape of the room and where the speaker is placed to better fill the space with even, quality sound. In theory there will be a wide or infinite "listening spot" for HomePod. You may not get stereo separation (although this was one of the original use-case scenarios for beam forming among researchers, stereo with one speaker array, so it may happen) but you also won't (theoretically) get dead spots or have to tailor your room for your speakers.
I think the problem is that most people haven't gone through the trouble of optimizing a listening space so having a speaker do it for you doesn't seem all that magical. Most people probably won't even have the chance to compare it head-to-head with a comparable, traditional speaker. In the end it will just be described as a "really good sounding speaker". So despite being one of Apple's most truly innovative products in a while, success is far from guaranteed.
EDIT: Grammar
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You're right (there will never be any MORE information; everything other than what was put down by the engineer is interpolated) and wrong (it will sound better because with a properly set up surround system should open up the proper listening space i.e. there is a bigger area in your room where you can get all that stereo information in the way it was intended, not to mention you will get more primary sound as opposed to secondary sound aka reflections).
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That would be a nice option but it wouldn't (right now) play to Apple's strength. HomePod is my top pick for smart speaker right now but I honestly wouldn't buy a speakerless Siri assistant. Siri is at the bottom of assistants in my experience—not bad but lacking in many areas. Is it just voice access to Apple Music that you want?