How does limiting the hours of the Dinosaur ride prevent stress on the animals? I think you read the OP wrong!
As for limited hours ... DAK has always participated in EMH, with no animal problems at all. Early morning hours are active for the animals whether there are guests in the park or not.
However ... the question was regarding the Dinosaur ride.
That would seem logical, but it is also determined by where the rides are located in the park. For example ... Splash and Thunder Mountains are two of the most popular rides at MK, but they are not open during EMH because it would necessitate opening all of Frontierland as well. So, by keeping EMH to Tomorrowland and Fantasyland, it gives the rest of the park an extra hour to "wake up."
Since Dinosaur is in a totally different part of the park than the other rides / attractions open during EMH at DAK, opening it would necessitate having a whole different side of the park open for one ride.