The original law in 1516 was just mentioning three ingredients, barley, hops and water. Back then they didn't know of yeast. The beer was often fermenting by accident and the yeast in the air, that's the reason why many breweries were located near bakeries.
Discussion
Second very interesting fact is, that the brewing regulations were a response to numerous complaints about bad beer. The official beer price fixings themselves were a major reason for beer counterfeiting. In order to secure their profits despite rising raw material prices and different regional conditions, many brewers responded with poorer quality.
So what do we learn from this?
Say no to price fixation?
Good ole wild yeast. I enjoy a nice funky beer from time to time.
I was going to mention this but I see you already did. It was probably innoculated from things like wooden stirring paddles. I think "airborne yeast" is more of a myth than what actually happens.