Interesting thread- thanks for sharing! I was also under the impression that the story was apocryphal but had not seen it explained in detail. If nothing else, the story always sat wrong because if you go far enough back to where the water was dangerous, you get to pre-germ theory anyway and people didn’t know water could be dangerous. If you go further back than that, water wasn’t dangerous. Or you get to a period where people suspected water, but only a few people in some parts of some cities were affected so there would not have been wide-scale behaviour change. Water was only bad in cities and most people lived rurally until recently. I’ve seen people apply this story to all different periods of history and it never adds up if you dig into it even a little bit.
It’s also one of those ”just so” types of story that people love, and those are never true. 
