We’ve made reservations for 6pm one night in January at Tusker House but I see Animal Kingdom closes at 5pm that day? How can that be?
That seems to be a very good indicator that the park hours will be extended. At this point no one is confident that all the September hours are final.
So what happens the closing time holds and our reservation is after closing. Can you still dine while the rest of the park is closed?
I’d assume so. You can dine before the park opens, and if you’re IN a restaurant eating when the park closes, you still have something like 90 minutes to exit the park. Feels like you have the place to yourself! Many shops are still open, because … they want your money! 
I have a 8:45pm ADR for Thanksgiving night at Tusker House, Disney shows a 6pm closing this night so far. Either they are expecting to be open till 9pm or I will get an email within a couple of months telling me they won’t honor it.
Edit - spelling, its what I get for posting on mobile.
They will honor your reservation but AK will be open later than it is currently posted.