78F is not even particularly warm. Just dress appropriately and you will be fine.
(you could try to structure the program so that only low-risk healthy adults were in the voluntary reduction camp, but it does seem like a bit of a logistical nightmare).
Evidently you are not enrolled in the "smart thermostat" program described in the article, which allows the utility company to remotely raise your thermostat setting regardless of what you set it to. They apparently did that without any regard to the impact it would have on comfort and health.
Aagin, the point is the AC is definitely running if the room is only 78F in Houston.
Also I think people really overestimate the humidity level indoor. Even if you leave your AC off for 2-3 days, it will not reach 90% RH unless you leave all your windows and doors wide open.
The concern is more one of safety than one of comfort.