They can pretty much start writing now, and since there's a whole lot of authors, they can parcel the adventure workload.
I'm guessing that many of the monsters are already created (from ToH complete or Tome 4) and they even indicated that the spells will be pulled from existing products and modified to fit 5E.
If they are done by Oct. 15 with the creation of content and they print in the USA, the end of November (for the Holiday season) is certainly do-able.
Besides, we all know Chuck and Skeeter don't sleep, so they have more time in the day than most of us.