Just curious, did you challenge them at all when they said you couldn't have the discount? The only comment on dates in the Passholder Benefits Guide is "*For a limited time October 1, 2016 through September 30, 2017." And the only exclusion listed is in regards to some restaurants not being eligible; nothing about dates. They don't even have the generic "blackout dates may apply" statement that they would normally put on an offer like this. Even the 10% discount dining program (that has a few more restrictions listed), doesn't have a general blackout date statement. There are just a couple of non-Disney owned restaurants that have some blackout dates listed that apply only to them.
If they refuse me discounts on Christmas Day, I will definitely push back. I'll be polite, but I'm not above pulling up the PDF of the guide on my phone & asking them to show me the exclusion.