I kinda agree but think that you are overselling the signal generated. We have internal data that we can't discuss here that throws some doubt on the signal from whiteboard interviews of the type described above.
Perhaps the answer is that there is no systematic interviewing system that actually works well. It seems like trying to create one was the origin of all this pain, and the cargo-culting by companies that don't need a systematic process has amplified it.
Interviewing is an inherently social, inherently subjective process. Such things are highly resistant to systematic approaches.