Call me cynical, but if engine braking isn't enough to maintain a given speed when descending, how else do you think the vehicle maintains its speed?
The 10R60 in the Raptor seems to lock the TC at around 1,000rpm so if you are in 4L and 1st gear then it is really no different than being in a manual vehicle (in terms of engine braking). If that is not enough to keep you crawling down at a constant speed, guess what happens next.
So this past weekend I did some mild off-roading that had some steep descents. I can say that in 4L in 1st or 2nd this RR is worlds better than my Canyon. It does actually maintain speed almost as good as my Taco did. Not quite, but good enough