What you stated is a complete heresy as the fake advertising done by Nintendo at the time... taken from wiki
Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that creates an illusion of multi-directional audible perspective. This is usually achieved by using two or more independent audio channels through a configuration of two or more loudspeakers (or stereo headphones) in such a way as to create the impression of sound heard from various directions, as in natural hearing
So where and how could you ever say that a single loudspeaker can output stereo ?
Nintendo lied at the time to the consumers. The sound board may produce stereo sound aka two different channels but as they are mixed, the produced sound is monaural and you could not create multidirectional sound with one speaker.