I stand corrected: the benefit is indeed applicable. Something strange happened:
A member of the party checked in first, and was offered Bkfast for 20 euro (despite the booking being under my name and being a diamond). I checked in later and was offered Bkfast for 30 euro, which I challenged due to diamond status, and then the front desk explained that the restaurant being operated by a third party, they could not offer the free Bkfast benefit, but a 10% discount. I didn't give it much though, alltough the difference in Bkfast rate quoted did raise some questions when I commented it. But it all become even more confusing when I read the usual welcome letter from the manager on which the diamond free Bkfast option was mentioned.... So this morning I asked the front desk again, the manager was puzzled and confirmed that free bkfast was of course available as a benefit. So we were able to enjoy a complimentary bkfast.
It left us wondering if there is a problem with training, if some of the front desk people are playing games, or what is the real issue. In any case, it didn't give us a good impression.