But it also says that the promotion only runs at "participating retailers". Is Digitec part of it?

AMD: *Eligible products include AMD Ryzen™ 7 and AMD Ryzen™ 5 desktop processors with 2000 series model numbers. After purchase, the product must be installed on the system for which the coupon code is redeemed. Offer only available at participating retailers. 18 years and older only. After purchase, voucher code must be redeemed by 1 June 2019. Residency and other restrictions apply. Full terms and conditions: https://www.amdrewards.com/terms.

Promotion period begins on 9 January 2019 and ends on 6 April 2019 or earlier when all voucher codes have been used. Eligible AMD product must be purchased during the campaign period. The offer is not valid for regions where it is prohibited.

So you should be able to register your purchase and get the code afterwards.