This is interesting to me, because my intuition would be that it doesn't matter whether the calories are going into microbiota or fat, it's still going to end up as weight in your body (gut bacteria still add mass).
Also adding to what another person said, the difference here is high fiber vs. low fiber. And I think the discussion around nutrition in general is moving away from weight being a primary determinant of health but a symptom of health. I.E. we shouldn't be seeking to lose weight but to eat healthily, high fiber, whole foods, less processed junk, exercise frequently, and weight is a trailing measure of our success in that.