As far as I know, someone noticed something fishy and told the final interviewer their suspicion to be sure. Like the way the candidate would only explain stuff after it was written. If you're on a video call and observant, I think you could tell if the typing didn't sync up with what you saw. I imagine it might be hard to notice at first, but if someone suspects it I would think a skilled interviewer could throw a few wrenches in their plan to see how the respond.