Millet rye bread

Hi!

I wanted to try the Millet Rye Bread recipe from "Secrets of a Jewish Baker" but the ingredients for the sponge just don't seem right. It cals for water, millet, yeast and... rye flour? After mixing the ingredients it tells me to wait until it triples in volume. Is that even possible with those ingredients? As you notice there is no white flour in there. I want to bake this bread because I would like to expand the list of ingredients I use for baking and because I never baked one like that before. Thanks for any insights.