Usually I think of leavening as either yeast or chemical (baking soda / baking powder). On these sourdough pretzels yeast and soda are both listed. Why would a baked product need both leavening agents?
For pretzels or "things with a pretzel crust" an alkali is used in the water they are boiled in before baking. That may be food grade lye (sodium hydroxide), food grade sodium carbonate (commonly found as washing soda, but not food grade), or sodium bicarbonate (baking soda - can be converted to food grade sodium carbonate by baking it.)
Not used for leavening, though there's also no particular reason you could not use both yeast and chemical means for leavening.