Interesting but in one respect not really correct:
"Another way to look at it is that when you need the power, you need to spin a motor really fast because that is where it's most efficient and can make the most power,"
An electric motor is not more efficient at higher rotation speeds:
"By inserting resistance into the rotor circuit and varying the resistance, it is possible to obtain the maximum torque at any desired slip or speed."
Yes and also no, first that the Semi motor isn't induction, second, yes, you can have peak torque at almost any speed other than once you get into the field weakening region, which means you are spinning the motor faster than what you system voltage would allow using some black magic that manipulates the shape and phase of the magnetic field in relation to the rotor, but you can't have any power at any rpm, you can have the same power for various torque/rpm combinations as long both are within the motor operating range
But what is wrong is that torque is not power, power is torque * rpm. Let's use a Model 3 motor efficiency map and the Semi 80 kW power needed to cruise a flat road at 55 mph as an example since the Plaid motors efficiency curve is likely similar if not better
There are infinite rpm/torque points that achieve 80 kW of power, some examples and it's efficiency
1000 rpm and 760 N.m - Outside of the rated range and operating sub 70% efficiency
2000 rpm and 381 N.m - ~75% efficiency
4000 rpm and 190 N.m - 92% efficiency
And so on. Now to the peak power:
1000 rpm and 475 N.m - 49 kW
2000 rpm and 475 N.m - 98 kW
4000 rpm and 475 N.m - 196 kW
This is just to show that if you need more power, you need to spin the motor faster, up to a point
I went back and counted the gear sizes from the Semi unveil video, and the ratios are as follow
Acceleration drive units: ~18:1
Highway drive unit: ~12.5:1
So at 55 mph and 80 kW, the highway drive unit motor spins at 5500 rpm and 138 N.m, operating at 94% efficiency, and at 70 mph and 125 kW, 7000 rpm and 170 N.m, operating at 95% efficiency
Power numbers from here
https://www.motormatchup.com/efficiency?id=607714741f6c21a772fbf034
So if you want power to climb a big hill, you need a gear ratio that puts the motor at or close to the maximum peak power point, in the case of the Model 3 motor from the plot bellow, it would be anywhere from 5500 rpm to 18000 rpm and the respective peak torque. Keep in mind there is other loses to take into account when setting a gear ratio, bearings, gears and so on that quickly adds up, so the optimum point will be narrower than those rpm ranges
In the Semi case, the acceleration drive units spin at 8000 and 10000 rpm for 55 and 70 mph respectively