I think what it is doing is measuring voltage directly across the battery and using a Hall sensor to monitor the current to one or more batteries in a chain (the current will be the same in them all as they are in series). That allows power to charge the battery to be measured and power drawn from the battery to be measured but I can't see how it can measure internal resistance unless there is a change in current. The web page shows the batteries disconnected by a switch so that would prevent any current reading anyway.
As I see it, the batteries must be discharging before internal resistance can be measured that way and the calculation would have to be made by adding time to the equation, in other words monitoring voltage drop per second based on the current being drawn. I would think that measures battery capacity more than internal resistance though.
Brian.