Can you explain that? During the initial constant current phase, cell voltage rises while current is held constant, until it reaches the cell's max. Then the voltage is held constant while current trails off.
Both imply in increase in internal resistance during the charge cycle.
If the battery were a resistive load, your statement would be true. The battery is not purely a resistive load.
The power in the charge goes into shifting the ionic chemistry of the battery to a different state. That changes the "resting" voltage of the cell. Which is quite different than changing its resistance. You can measure the resistance of the cell during charge; many chargers do this. A typical 18650 style cell has a DC resistance in the 110 Micro-Ohm ballpark when healthy. Tesla doesn't publish specs... it is going to be in this order of magnitude area. 110 Micro Ohms is 0.00011 Ohm.
The pack is arranged in a serial/parallel arrangement that I can't quite track down as I type this... the parallel part reduced the effective resistance and the serial part increases it. Any reasonable assumptions still end up at tiny fractions of an ohm.
Charge current is not going into resistance (in fact, any charge current that DOES go into resistance exits the pack as heat). Charge current is going toward changing the status of ions in the pack.
So... why taper? As stated with many different phrasings above, pushing too much current during the latter portions of the charge cycle raises the cell voltage to the point that PERMANENT DAMAGE occurs to the ionic chemistry of the pack.
If you want a physical metaphor, think of charging as being like pushing trillions of tiny needle points against a stretchy plastic film. If you push too hard, you will push some of the needles through the film, and this can never be undone. In the initial part of the charge, when the film is quite slack, you can push all the needles fairly hard (lots of current). As the film begins to get tighter and tighter (and you can measure this tension, it is called voltage), you need to start backing off the push (current) when the film gets near break strength (voltage gets to 4.135 per cell at 20C).
TL;DR - Resistance has nothing to do with tapering charge. It tapers because exceeding the critical voltage of the ionic chemistry of the cell, even a little, damages the chemicals in the pack, permanently.