Sure, I can see the "brick" using more than 13 kW on the highway... but that moves you up the performance chart to a more efficient operating point, so the energy consumption from the battery doesn't go up quite in proportion. Heat load in the motor is due to losses, which means input power multiplied by inefficiency (e.g. at 85% efficiency, 15% of the input power is turning into heat). At 30 Nm and around 6500 rpm (so, 20 kW) the motor efficiency is up to 92%.
In rough terms:
car: 13 kW at 100 km/h -> 20 Nm @ 6500 rpm @ 85% efficient, so 2.3 kW (15% of 15.3 kW input) of heat from motor
truck: 20 kW at 100 km/h -> 30 Nm @ 6500 rpm @ 92% efficient, so 1.7 kW (15% of 21.7 kW input) of heat from motor
Churning along at low load is mostly an exercise in generating heat, which is why some recent mid-engine hybrids with front electric drive disconnect the front motor at high speed. It wouldn't make sense in a battery-only EV, but does if the other wheels are driven mechanically by an engine.
Up at highway speeds, the same power is available regardless of engine speed, as long as you don't to past 9,800 rpm. That means that a moderate change in gearing won't make any difference to the power (or torque) to the wheels. If you gear lower (more reduction) or use tires of smaller rolling radius, the torque available at low speeds (below 2700 rpm) will improve, but do you need that?
I wouldn't consider smaller tire diameter, mostly because this is a truck.
Although the Leaf is probably a Versa structurally, and a Versa isn't a heavy-duty vehicle, I doubt that structure is the towing concern. Although the rear suspension doesn't have a lot of spare load capacity, I doubt that's the biggest concern with towing, either (and isn't relevant to the use of a Leaf powertrain or even front suspension in the VW truck). The big constraint is probably that continual high power demand is an overheating concern for the motor, battery, and electronics.
The gearbox is really simple. It will be designed to handle the motor's torque. Handling high torque continually should just be a matter of keeping it cool enough. That might call for circulating oil through a cooler, but simple gear transmissions like this, with no clutches or hydrodynamic components (such as a torque converter) rarely need an external cooler. You'll see a cooler occasionally on a race car's rear axle, but that's due to hypoid gearing and the clutches of limited-slip differentials, neither of which are found in the Leaf.
I'm not disregarding the importance of adequate hardware for the job; I'm just saying that the Leaf motor shouldn't have enough power or torque to hurt the Leaf gearbox, given adequate lubrication and cooling.
