The principles are different. Here's a quote from Sound on Sound:
The way in which our ears interpret sounds from loudspeakers is inherently very different to that from simple headphones. Stereophony is an auditory illusion — much as watching 25 still pictures each second in rapid succession creates the visual illusion of naturally moving images. When listening to a loudspeaker, its direct sound inherently arrives at both ears, and if the speaker is positioned off to one side (consider the left-hand speaker in a stereo pair, for example), then its sound will reach the closer ear slightly earlier than it reaches the more distant one. The fixed time-of-arrival differences for each ear combine with the variable amplitude differences encoded between the channels of the stereo audio material, and fool our sense of hearing into perceiving different time-of-arrival information for each reproduced sound, and thus discrete directions.
Headphone listening works in a very different way. When wearing headphones, each ear can only hear the sound from its own earpiece — there is no natural way in which the sound from the left earpiece can reach the right ear, for example. As a result, the recorded amplitude differences between the left and right channels don't create the required time-of-arrival differences. The consequence is that most of us perceive sounds coming from inside our heads, spaced roughly on a line running from ear to ear.
But it should be possible to create a good stereo-like effect with the HomePod. Time will tell how well it works.
