It does seem like a convenient "just-so" anecdote, and I've never experienced or heard of myopia just going away like that, but I also don't think it sounds completely implausible.
I'd imagine it's also less noticeable in scenarios when you don't need to read smaller letters from a distance as much (ie: when you are playing outside rather than sitting in class).
Also in bright light your pupils are constricted so depth of field is greater. In dim light your pupils are dialated and depth of field is shallower. Same principle as a camera lens f-stop.
So the perception that you see more clearly in bright outdoor conditions could be true.
