This is because OZO Playback can use the earpiece speaker as one of the speakers, so even with mono speakers like most of HMD's Nokia phones it's possible (I believe) to get stereo playback! It might not be very good, but it would be something, right?
I would love it if HMD could bring these feature to existing Nokia phones, however I think that hope if forlorn considering that HMD aren't even fixing broken features with the current phones and are now even starting to lag behind with security updates.
Anyway, if not current phones, maybe HMD can bring this to new phones? I hope so.