Apparently my coworkers interviewed someone a while back who got pretty far into the onsite interview panel before they realized the candidate was not the same person typing out the code they were seeing on CodeBunk...
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.