About polyphony. The Play’s MIDI tracks are polyphonic. They take more than one note per step. They are not monophonic. MIDI record multiple notes on the same step via external controller is a wish that has been declined. Despite of the lack of this feature, the tracks are still polyphonic.
About playing chords with an external keyboard. Selecting notes and chords via knobs isn’t as convenient as playing the chords with a keyboard controller, but users can still enter multiple notes per step. And those who aren’t familiar with a piano keyboard and chords/scales are probably happy with selecting just the root note and then the chords by their name.
Taking a step back… what are the real-life situations where a user can’t record the notes they have in mind using the 20 pre-programmed chords or the Piano Roll? What are the situations where only an external MIDI keyboard could save the day?