Here is the study. I don't follow this doctor so don't know anything about him but this is the study that Dr Paul Mason quotes in some of his lectures. It is like a couple minutes long. It is also the study that the guy I mentioned (Physionic) to run the study down. He brings out its shortcomings like small sample size, etc. But he never really says it is not true nor does he have any other controlled studies to refute it (most studies that "associate" high fiber with health are only food questionnaire studies that often have lots of confounding things and can not determine causation - kind of like you see fire trucks at fires so fire trucks must start fires - association, not causation).

 https://www.youtube.com/shorts/1Svi5fOQohM