My own child wasn't diagnosed until that age and later, so I couldn't say, but I wonder if it's possible they're talking about the natural jump NT kids make in communication, which tends to make kids with a delay appear like they are regressing? I've never heard of an actual regression in a kid that age.
DS certainly had a nose dive in behavior around 8-9 years old because that was when the pragmatic speech deficit was exacerbated to the point that he couldn't communicate effectively, but it wasn't that he was regressing - it was that the other kids were developing. This is, actually, quite common and is (according to our psychologist) the primary reason why kids with AS start getting their diagnosis at that age.