So people extremely knowledgeable in a domain don't make mistakes in that domain? Or are they just "one notch too low" on the continuum- enough so that they make a mistake? I just don't think that holds.

Mistakes aren't always made due to incompetency and extremely competent people still can make mistakes.

I think most of the "seven factors that lead to stupidity" could still affect someone very competent: https://fs.blog/2019/01/how-not-to-be-stupid/