He's not allowed to discuss future plot points, in his own words. Technically this is a past plot point, though I heard that Greg got tired of answering questions because people didn't always like his answers, so he might not answer it anyway :\ As many of us witnessed on these boards in the eve of 2015, Greg has received a lot of criticism for his responses to canon questions. I can easily understand why he would grow tired of the process of answering questions and bracing for the reactions to it.