Watched video with sound, and I think I understand what you were getting at--that it's the positioning of the tip of the spout vis-a-vis the water level which guides the pour.
I would also postulate that the wider diameter of the base in contrast to a narrower tip keeps the pressure of the flow more consistent than a spout that is the same diameter from base to tip. That would create just enough back pressure to help control the flow. I think both elements are needed.
The analogy I thought of is breath control for musicians. If you just breath in and push out, it's like that big spouted pot in the video. Everything pours out fast, goes wherever it goes. When breathing properly, you have a big base (abdomen), narrower diameter at end, and a lot of back pressure to control how much air is released and where the air goes.
I really thought that this thread was tending toward überobsessive, but seeing John's video and knowing how my Hario and Bonavita pour, I am amazed at the difference in the ease of control.