I don't think it's a myth, if you make a sourdough starter with the same brand of flour in five different locations around the world you will get 5 different collections of organisms. For instance, where I live you get instant acetone with any attempt at wild yeast collection, airbourne, fruitbourne, whatever. But 2 hours drive away and your in the berkeley san francisco golden zone where it's hard to get a bad starter.
that being said, I am sure in lambic breweries the equipment, building, barrells and everything else is steeped in the house organisms and that is part of why it works as well as it does.