They can only make the battery 99WH, and still have it allowed to go on an airplane. The battery is already ~83WH, so sadly, there may not be much to gain there.
While it's technically true, it's worth noting that part of the weight & thickness savings in the 2016+ MBPr was from simply reducing the battery from 99.5 Wh to 76Wh. I strongly dislike this change.
