It's a gloss: your intestinal microbiome is maintained because bacterial biofilms in the intestines constantly grow, but are also constantly scraped away by fibrous matter travelling through the intestines (the balance is sort of like bone remodelling.) If you eat only fiber while otherwise fasting, the scraping will occur without the regrowth. Combine this with a course of antibiotics beforehand and a course of probiotics (or a foecal transplant) afterward, and you can fully replace your microbiome.