eh, that's a lie perpetrated by the Cantillon brewery.
Yeast is everywhere. leave some milk out on the stove and you'll get yogurt in a few days.
say, why does that fruit on your counter start smelling a little acrid after a few days? that's natural fermentation.
It's a myth that keeps them relevant. BUT, they make awesome, awesome lambics. it's a cool place to visit, too.
You are right...yeast is everywhere. But there are different strains of yeast and the one that gives a real lambic it's characteristics only exists in that part of the world. I can buy a yeast strain similar and make a lambic but not in the way it was originally made.