Why does Tesla recommend such a small gauge wire for their chargers compared to this chart I found online?
That chart is from BlueSea - they make marine DC power system components (mostly 12V and 24V). Voltage drop effects are completely different at 12V vs. the 240V used by this EV charger. Disregard that chart.
Plugging the actual scenario (240V, 6AWG, 40A load, 150 feet) into a voltage drop calculation indicates that the worst possible voltage drop will be only 1.98%.