Rising without yeast

I'm on a food list, and there's a slight controversy about "sourdough."

A- asked for bread ideas for someone who can't eat yeast.
I suggested matzah, which has none at all; it's just flour and water, baked very quickly, no time to rise.
B- mocked my answer, saying that, flour and water left alone will ferment and rise, becoming yeast.
I decided to check it out on the internet!

The first site I found was a quest to make "real" San Francisco Sourdough Bread. According to his instructions you need a "special sourdough yeast starter." It's different from regular baking yeast.

In this site I found a more scientific explanation of the various yeasts and how they thrive. Sourdough does have yeast. I understand that some people can digest that bread more easily than the regular type. But it's incorrect to think that it is yeastless.

Yeastless, no, but yeastless, yes. Now I guess that doesn't make sense at first. Simply, you don't need yeast to make yeast, but it takes more than the time the matzah dough is sitting. Here are some simple instructions how to make your own sourdough yeast starter. The secret is that there is yeast in the "air," and if you leave the dough to sit under the right conditions, it will ferment, and you will have yeast. Also, you have to give yourself at least a week until you have sourdough starter for baking. After that you can become more spontaneous.

But Passover matzah doesn't have yeast, since it's prepared too quickly.