Ya know, I have been wondering the same. I got my bike in winter, motors love cold.
I recently rode at 92 degrees.... I thought it might cut back in power as the ambient temp is important for electric motors, and maybe it did, but I didn't notice a difference (I am riding turbo all the time, and I can feel the motor does get hot if I ride non stop with no break in the ride).
DJI could have approached this a couple of ways...they could have a thermistor to monitor the temp of the winding, (which I suspect is NOT the case, or they would have more confidence in extreme heat instead of recommending otherwise).... or they could be protecting the motor via firmware in the controller with a current limit profile over time. That would assume some ambient temp range (I'd guess, more likely)
Regardless, when the outside temp is Hot, it just reduces the amount of current the motor can draw over X amount of time, without thermal overload. In cold, its the opposite.... it could put out more (if the controller has the capability.)
ITs really hard to say, because we don't know how hard the motor is being pushed relative to it's peak continuous output capabilities. Continuous is more important that Peak, with high temp. I'd say maybe just ride on Trail mode when its like 110 degrees out...that will reduce the current draw / average temp of the motor over time.
Man when it's that Hot, time for a swim!
Cheers