In case of a positive test result for yeast you recommend sourdough as a suitable alternative. How could this be if during fermentation yeast is produced? Isn’t it the same with vinegar?

Pure sourdough fermentation is mainly done by lactobaccilli, a bacterial strain, and hardly any yeast. So if yeast isn´t added on purpose, sourdough bread should contain only small quantities of yeast. The same with vinegar: Its final fermentation is done by bacteria, acetic acid bacteria mostly from high percentage alcohol, not yeast.