Yeah, humidity is the thing that makes the difference. 85 and low humidity is a dream come true compared to 90+ with high humidity. I live in Eastern NC, and it gets absolutely brutal here in the summer. And unlike spring weather when it still cools down overnight, it stays muggy and miserable pretty much around the clock. The only difference is the sun isn't frying you at night.
BUT, you'll be at Disney. Swimming pools, water parks and rides, and plenty of air-conditioned attractions and restaurants. It's not like you'll be out there doing yard work all day. You've got plenty of options for getting cooled down. And there will most likely be a thunderstorm every afternoon/evening to at least cool it down temporarily. Sometimes when the sun comes back out after a storm, it's downright steamy.
Just know it's a different kind of hot. Like a hot, wet towel is wrapped around you all the time (including your face).