Welcome to the Community Forum.
The promotions at promotions.fairphone.com have always been dealer-only. Getting the Fairphone into the shops of major outlets like phone operators/providers and online stores has been an important factor in Fairphone’s growth in recent years. Most likely it’s not just convincing words that made these outlets distribute the Fairphone, but also deals like the one you are referring to. These deals are an incentive from Fairphone to them to keep offering the Fairphone, and if Fairphone would offer the same deal at the same time, this might undermine their willingness to keep offering the Fairphone.
Then again, Fairphone also runs deals and promos from time to time that are exclusively available at their own shop, see https://www.fairphone.com/en/legal/fairphone-promotions-and-offers/
P.S.: If you can wait a bit, you might actually find a similar deal in the Fairphone Shop – every now and then, Fairphone has offered free “add-ons” in their own shop, too (see the link right above). But I have no insight when this might happen. Here’s an overview of past promos, either at dealers or at the Fairphone shop only: Fairphone discounts & deals 2020, 2021, 2022, 2023