It's not only a lack of fat that would cause dryness, but also the lack of sugar, since sugar retains moisture - maybe the combination of both low fat plus no sugar was the cause.
Was there any other source of fat other than the peanut butter? Sometimes you can use unsweetened applesauce to replace fat. Can you post the recipe?
"Strength is the capacity to break a chocolate bar into four pieces with your bare hands - and then eat just one of the pieces."