What does an OOB have to do with stacks?
I could stack 40 units together with an OOB, the only mechanics that refrained me from doing so were the stacking penalty and attack delay. However, when attacking France (over Luxembourg) for example, I always had roughly 40 divisions (plus HQs) stacked in one province, let 2 of those divisions carry out the breakthrough attack just to flood the hole in the defense with the remaining units afterwards.
In HoI4, it seems to me, stacking kind of depends on the "width" of the frontline you draw. If you assign 40 divisions to a frontline of the "width" of 2 provinces, they will obviously stack (maybe 20 for each province). If you assign 40 divisions to a frontline of 20 provinces (like the German-Polish border) there might be about 2 divisions per province. This has absolutely nothing to do with an OOB, so I do not understand this concern and don't get several people bringing it up.
Plus, as you could see in the stream, even if you order all the troops from the German-Polish-border-frontline to attack Warsaw, they will advance on all fronts. The green blinking provinces pretty much indicate where the troops are going to move, which is awesome. Encirclements are really easy to manage now as well. Lets say you draw a circular frontline from Dresden to Ostpreußen, the divisions at Danzig probably won't advance at all, whereas the divisions in the south would make their way up north-east. It is a beautiful design in my opinion, especially considering you can always split the front, have certain units perform independent moves etc. Units will only stack if the player him-/herself orders the units to do so.