I'm no battery expert, but my general understanding of physics and chemistry tells me the current path isn't the only source of heat generation when it comes to supercharging. But yes, I'm just speculating based on the graph and also based on the fact that Tesla didn't mention any charging speed benefits with their new battery.
Also, you can't use wire physics as an analogy to thermoelectrochemical processes within li-ion batteries.
Sure, resistive losses are not the only heating factor, but they are a factor (that increase as the square of current). The electrodes are wires (very thin and wide wires), so one can use that analogy when discussing the resistive heating effects. Reducing that effect reduces the heat generated. Thus, if the cell charge rate over a supercharge session was thermally limited (as you mentioned), then it will increase the overall charge speed (note that peak charge rate is partly lithium ion transport limited, but also resistence limited).
To claim the 21mm tabless charges at the same rate as the 21mm tabbed means that none of the changes in chemistry, manufacturing, and packaging provide any improvement. The energy density increased 5x (kWh), and the power density increased 6x (kW). Would that not imply a 20% increase in current handling (both at a charge and discharge level)?
As to not mentioning this impact more directly, they also did not mention they have had cells in vehicles since May, so omission is not an indicator of anything.
Also, there is the complication that V3 superchargers max out at 250kW (per Tesla).
Tesla V3 Supercharger Test: We Find Out Exactly How Fast It Really Is
Model 3 already hits 250kW charging. So no cell changes will impact that peak number on current equipmemt (nor is the new taper likely determined). However, the new cells can reduce the taper in the ways:
1. Less heat generation (shorter electrical path)
2. Better heat rejection (direct electrode to can interface)
3. More uniform voltage
To expand in #3: a critcal parameter in charging is the voltage potential applied to the two electrodes. If this is too high, you get litium plating which permanently reduces capacity. V=IR, so as we move further down the electrodes from the tab, the voltage drops and so does the charge rate. However, the max voltage is limited by the higest voltage in the cell (at the tab junction). Thus, as charge progresses, the cell charges from the tabs outward. Reducing voltage drop increases the rate at which the total cell can charge without violating the voltage or amp/mm^2 current loading.