Sourdough still contains yeast - just yeast that is naturally occurring in the atmosphere where the starter is made.
Yeah, natural yeast is not listed as an ingredient (or else it would be an ingredient in everything), sourdough only has 3 - flour water salt.
The starter that contains the yeast is just fermenting flour and water.
If you see an ingredients label that says yeast as an ingredient it’s not real sourdough.