It’s a wavetable created using additive synthesis. Read @Ess’s post, it’s quite informative if you want to know more about the Chord machine.
I love it, by the way. In my view it’s a unique way of adding some harmony to my songs.
EDIT:
Couldn’t find the actual post of @Ess I was referring to, but here’s one instance where he talks about it. I might find the more comprehensive post later.