"Fall" has several definitions since it is used in different ways. So, yes, the astronomical seasons are based on the solstices and equinoxes, but the temperature peaks and averages lag behind. People feel the temperature, they don't feel an equinox.
Then you have the human social order adjusting to seasonal changes. The equinox may not be until Sept 20, but for all the families with school children, fall is when school starts after the summer break... the fall semester often begins in late August. Business that deal with vacationers often regard Memorial Day weekend the beginning of summer and Labor Day weekend the beginning of fall.
Then you have different latitudes and elevations experiencing season (as in weather) changes at different times. Northern New England and the Rockies are already experience leaves changing. For them, fall, the weather season, is already here.
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A New Englander visiting WDW now would not regard fall colors to be out of place.