They are (besides the Monotron Duo, which has a second oscillator (but no way to play it paraphonically without hardware hacking, you can just tune it differently)), but this is the Monotron Delay, which has, well, you can probably figure that out.

Yeah, the monotron is monophonic. But you're ear is right on because it does sound like multiple synths - I'm actually augmenting the monotron a bit here using another device called the norns [1] with a script I wrote [2] which converts the initial drone note into a drone covering six different pitches at various volumes (sort of a super saw). After that augmented drone you'll hear an arpeggio which is pure monotron (plus its delay which adds a little polyphony as mentioned).

[1]: https://monome.org/docs/norns/

[2]: https://llllllll.co/t/barcode