Fermentation is not the principal pathway by which bacteria process sugar. In particular, sugar is a key raw material for growth, when available.
Point is, the products of bacterial metabolism of sugar in the human gut tend to cause issues, as in lactose intolerance, or the more rare sucrose intolerance. Hence my request for a citation on fibre delaying absorption of fructose so that "bacteria get most of it".
I am only passing on what Robert Lustig, who is in a position to know, reports. You would have to ask him.