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.
I know (I'm British) and the US doesn't have Autumn. They are the other way around.
In the comic, the US has both, with Autumn in the brief period before Summer ends and Fall starts. The alt text is saying the UK has the two the other way around.