Since the Winter for the Ages discounts are created before the game Updates are released, new content is usually not eligible for discount. (This is the same reason why Cold War Age wasn't discounted last year when it was introduced during the Winter for the Ages, but was subsequently discounted the following summer.)
Not eligible for discount - that's really weird explanation. Perhaps you could pass on this pseudo code to your developers: IF building age = CWA, THEN cost = cost*0.5
Yeah, it's that simple.