"It could be a thing," Strange points out. "You're going to radically change the magic of your world: it only makes sense that the spells you could or couldn't cast would change with it."

He doesn't really know how aer might be manipulated for scrying purposes. But it seems awfully silly for Rita to declare something 'not a thing' right before she goes back to her world and changes the entire system of magic. It could be a thing.