I spent years in the motor fuel ethanol business and never heard this. The DDGS (distillers dried grains with solubles) is mainly a source of protein since the starch has been removed. You fatten a cow on starch, not protein. DDGS is, however, a fair replacement for soybean meal.
From a North Dakota State University article:
But, despite its fiber content, distillers grains mostly consist of small particles, so distillers grain coproducts are considered to contribute less than 15 percent of physically effective fiber to the diet. [Fiber] from long particles is necessary to stimulate rumination health.