Do you see how much metal on the battery case, which serves as a somewhat massive heat sink, preventing the point of soldering from reaching the right temperature?!
That's exactly why you need the high heat. It solders the bar onto the cell plate before the case can absorb the heat. That keeps the battery from getting hot.
Have you ever built battery packs before I have been doing it for at least 3 years for my RC trucks.
These are heavy duty packs that need to take a pounding.