A common misconception. It's actually caused by the nauseatingly high volume of sugar in the five gallons of sweet tea they drink each day.
If the spoon doesn't stick up straight after adding the sugar, it's not sweet enough!
Note: the proper way to drink sweet tea is about a 1:10 ratio in my not-so humble, non-Southern Floridian brain. Ask any Southerner: only north Florida is part of the South. In my Floridian opinion more than 1:10 ratio is icky!
That said it would be interesting for them to research heat conditioning: if you heat condition yourself the weather still feels terrible but you can still think even while running in the full sun. It takes about 3 runs to trigger the heat conditioning in my experience. Most folks hate heat conditioning but it's actually not terrible. Just a little dangerous because heat stroke is very much possible.
Running in 85° sun-rain, however, just broke me the other day. So mileage may vary...