I don't know - I have seen lots of zero interest offers - but never from Affirm. I have a 750+ credit score and last time I looked at Affirm for something (a glowforge I think), interest rates offered were above 21%!
I’m guessing this was probably an arrangement between Peloton and Affirm to where the former paid some fee to the latter, and all of that is absorbed as marketing overhead just to get more people buying bikes.