That "fact" is not always true. We rode it on opening day, in the heat of the afternoon, mid-90s air temp. The skyliner stopped twice - once in the route from Epcot, the other as we were arriving at HS.
During the first stop we noticed that there was still good airflow in the cabin - did not feel hot at all. Those of us in the cabin were commenting that we still felt the airflow even though we had expected it to warm up quickly, which it did not.
In the second stop there initially was no airflow, but then some air began to flow. It did warm up a bit, but still was cooler than the outside air. Don't ask me how... but that is what we felt. And, we had no problem sitting either in silence or chatting with other guests. It was my wife and I, a single rider, and another couple.