The cabins don't warm up like cars. The wrapping the gondolas use prevents that. When stopped, the feel like temperature inside the cabin starts to feel like the feel like temperature outside in the shade. Every video out there featuring a temperature gauge inside the cabins shows the temperature doesn't spike inside the cabins when stopped in the afternoon sun, but somehow people keep believing these anecdotal stories about the cabins heating up like cars.
The simple truth is that if you can't handle long exposure to the outside temperature on a hot day, you shouldn't ride this on a hot day. You, also, shouldn't be going to the parks on hot days.