What it is
The cooking banana — a staple starch across the tropics, boiled, fried, or dried to flour. Green and unripe is a different food from the sweet dessert banana.
Why your biology objects
Unripe plantain and green-banana flour are among the richest sources of resistant starch (RS2) — starch that resists digestion and ferments in the colon to butyrate, feeding the gut exactly the way the thesis likes tubers to. Ripening converts that to sugar, so the prebiotic story is about the green form.