American here. I went to first grade with a girl named Autumn. Other than her I've never heard anyone use the word, though I've seen it in print enough to know elsewhere in the English speaking world it is used.
The joke here is that, because Americans do not use the term "autumn" in normal communication, someone might be led to believe that it had a special unusual scientific meaning.
Or as we call it in the Southern U.S., "Summer Lite," where the daily high temps are finally at or below 90°F a good bit of the time. We keep getting teasers of autumn, but it'll be a week or two depending on where you live. The heat index in Houston looks to be 105°F today, so autumn is still a little ways off. Ouch.