FOS can feed Klebsiella for example, but at the same time the acids and bactericides formed by the fiber can kill the bacteria. Therefore it is like still eating, which provides sugar for cancer cells, while killing off the cancer cells with other treatments. As long as the cells are being killed off quicker than they can grow there is not an issue.
As for feeding yeast again this is misleading. The fermentation of the fibers actually generate acids that control the growth of yeasts, and prevents Candida from converting in to its dangerous fungal form: