The flaw in the video is that the line on the V I graph is seldom linear.
The internal resistance is determined by many factors inside the battery and these factors will change as current is drawn. The thickness of the chemically active layers being one of the main factors in determining the internal resistance. The thickness of the active layer will change depending on how much current is being drawn from the battery.
There are batteries that do have a near linear V I graph over a small range of current but these are designed to be used as voltage reference sources and not for supplying power.
