About the only gondola system I've been in that's going to be similar to the weather conditions at WDW was the gondola system at the 1984 World's Fair in New Orleans.
http://neworleanshistorical.org/items/show/563
It was a ride 2500 feet across the Mississippi River, then back another 2500 feet if you didn't want to get off. The gondolas held 6 people and had no AC, and got up to a height of 320 feet. The view was amazing, and if you caught it at the right times of the day, actually refreshing from the river breeze. But they were hot as heck during daytime, humid and if it was raining, you almost felt like you were swimming from the moisture that would be circulated in. Also, they could get darn cold. After the fair, they tried to keep it going as part of the city's transit system, but it didn't last long. IIRC, no one died or suffered heat stroke or the like on the gondola, but there was lots of chatter about how much fun it was and how miserable it was, all by the same people.
A huge difference is that the 1984 gondola had only 2 pylons, each thousands of feet from the other, so that the Skyliner system should be faster than the 1984 Fair system was, plus there's been 34 years of technological advancements in the interim. So, I can see that if the speed is sufficient, that the ventilation system might well work as advertised.
And while many of us are accustomed to thinking of the monorail and the buses that the gondola system is meant to supplement/replace, WDW operates transportation without AC every day with the water transportation fleet. I know that nothing on Seven Seas Lagoon/Bay Lake has AC for guests. It's been so long since I've been on one of the Friendship boats that I don't remember if there's AC on board that fleet. But once the ferry boats start moving, that breeze sure feels nice.
Another analogy that a lot of people may have experienced: Going to stay at the beach, and the condo/place you're staying at doesn't have AC in the elevators. But I do wonder about whether an aftermarket heatpump fan might need to be put on the roof at some point (and yes, ha-ha, I did see the photoshop of the window unit stuck onto one).
So, I'll give Disney the benefit of the doubt that they and the engineers and designers know what they're doing.