I wonder if that's actually true, but I don't want to know it isn't

I doubt you'd be boiling the wort at a marketplace, even for small beer you're looking at a week or more of ferment.

Selling it from a tun at the market, having a characteristic dress (and probably call), is very consistent with European retail traditions. "Fishwives" were famous for their shrill, sharp calls for example.

The modern equivalent I guess is the ice cream truck; they pretty much all have the same livery and play the same recorded organ tunes. Just imagine that approach for every product category.