That's what's nice about Theory.... it can be proven wrong.
No matter what they tell you, unless they are doing it to solve the same exact problem, it doesn't neccesarily apply directly.
I think gearing the motor correctly, and driving it correctly are the biggest factor. Under gearing can cause heating problems as well. You end up "firing" a lot more often and the motor see's crazy RPM. By under gear I mean to small of a pinion, not the final number.
I've had some offroad drivers run a Motor that is "too fast" for the track, gear it normal, and then turn the EPA down. They report good temps and good feel. But this works because the loads in offroad are much different. Much less traction, so you don't "hammer" on it.
In On road when you have bit, the car setup and how it gets through the corner is super important. I've been to races for years, with regular motors, you can "tell" when the car is good, the motor stays in much better condition. With BL, you end up with no "signs" from the motor other then the temperature.
You're right, if the motor is going slower it's not working as hard. But you have to factor in the Load Now. Put load on the motor, and the RPM will change, as does the amount of current.
So if you're overgeared, and you apply full throttle or 7/8ths throttle, the load on the motor is still the same, its still trying to pull the same gear, just doing it with "less power" so it's making it worse. You see?