Buy a Ryzen 5 5600G or Ryzen 5 5700G between 5 August 2021 and 25 September and get the game Warframe for free....

Most likely, "promo" means a promotion code or a bundle of Warframe. I strongly assume that the "Sister of Parvos riptide pack" is meant. The update came out at this time c.a., so it would make sense.