I wonder if it's a case of soluble vs insoluble fiber. Insoluble fiber is often described as speeding up transit through the intestines, which might leave microbes less food to eat, or increase the microbes being driven out before they can establish.

I had a similar thought.

I had a laugh years ago when I realized that jam and jellies are full of soluble fiber and my entire childhood my parents were sneaking fiber into my diet. My whole childhood was a lie!

Microbes eat fiber. That's what a prebiotic is.

That's the kind of garbage results you get when you run a study with only 20 participants. Also they changed to both mediterranean and ketogenic? Wtf

Yeah, I thought fiber is literally food for gut microbes. WTF?

It was a 2 week study, and diet was only changed for a week. The time period was probably long enough for existing microbes to decline due to environmental changes, but not long enough for new microbes that favor the new conditions to move in.

If you've ever dramatically changed your diet, you'll know a week is not long enough for the resulting GI upset to subside. It often takes a month or longer.