Not trying to be an obnoxious nit-picker (but maybe that be what I am doing) but that statement isn't an absolute.

I've ridden in snow that was not only that deep but deeper, but it was snow that was packed hard enough by the snowmobiles that had been using the trails that I rode on top of it and only sunk in a couple inches or so. If its soft and you sink in the full 12" then I would agree that very few sane people rides a bicycle in that stuff, can't rule out the possibility of some nut somewhere doing it though although I bet he/she has a heck of a time doing so !!!