I was about to type up a reply decrying the idea of Potter soft-opening in December and how that is never, ever, ever going to happen.
But... well... I mean, if you think about it:
1) This catches Potter diehards off-guard. They have been repeatedly told 2016, 2016, 2016. Unlike Diagon Alley, this will prevent the bottlenecking of people waiting outside a land that just isn't going to open that day. If you think they are some people so rich and dedicated to be able to stand at USF for nearly a month straight, over a dozen hours every day... well extend that idea to California. Especially LA. That would be triple the amount, easy. So as long as nobody tips off any Mugglenet users, they should be fine.
2) The fear of capacity slamming the parks due to Potter openingand not being prepared is null and void. It's Grinchmas, so it's already a formality. If anything, it would spread those tight crowds around to make things more manageable.
3) It would also rev up the crowds in Janurary and February, which is notoriously slow for USH. I still remember working those months... oh no wait, I kinda don't. Because in Jan/Feb 2013 I worked very, very little. Two weeks of January that year, even with calling in to nab a shift, I struck out and just never worked. Even my (awful) roommate asked me if I got fired because I was in the apartment every single day. After that, for another three weeks I only worked once or twice a week at best, for six short, short hours. (this is also the time in which I even put in some time for JPRA and Mummy) I was not very low on seniority, and the days in which I actually got to come in, Transformers was a walk-on literally the entire day. I mean, never a full RV. Ever. It was unbelivably dead. Needless to say, they may as well take advantage of a slow time, while keeping profits high and attendance higher than usual. It's a win-win for Universal.
So, still, grain of salt totally but surprisingly it's not too boneheaded or far-fetched of an idea. I do think the earliest that the land will have its grand opening is March,