"No sourdough bread is not yeast-free...."
So true. If you read the bible you will find that during the Passover the Jewish people ate only "unleavened bread" which if you've ever had it is a bread that doesn't rise and is more like hard tack. If ANY store tells you they are making sourdough bread without yeast they are liars.
The original sourdough bread from San Francisco did have a wild yeast in it. (http://www.sourdo.com/home/cultures/original-san-francisco/)
The sourdough bread was made from the resulting culture and then a small portion of the raw dough was always saved as a starter for the next batch of bread - by either the 49ers, or the bakers.