That's not really how internal temperatures work though so you shouldn't be worried. Our bodies are designed to regulate our internal temperatures regardless of the external temp and unless something is wrong with your body, your internal temp shouldn't vary more than 1 degree or so. For example, we still swim in pools that are set to temperatures lower than 90 degrees, but if our internal body temperature fell below 96 degrees we would be considered to have mild hypothermia and a degree or two more would mean we were in a life-threatening situation. Even on a blistering hot and humid Florida summer day, you would have a lot more to worry about healthwise if your temperature rose above 100.4 degrees on the walk from the car to the gates than not being allowed into the park.
If you have a temperature above 100.4 degrees, regardless of whether its due to COVID, that is a sign your body is not well and its probably best not to put it through a day at a Disney park anyways. Some might see it as an unfair or arbitrary or even paternalistic policy, but health professionals have suggested that cutoff because 100.4 degrees is an internal temperature that has concerning health implications no matter what the cause.