Yes, but it's not evolution. From that page-

> A study among expatriates in Nepal suggests that immunity may take up to seven years to develop—presumably in adults who avoid deliberate pathogen exposure.[18] Conversely, immunity acquired by American students while living in Mexico disappeared, in one study, as quickly as eight weeks after cessation of exposure.[19]

I suspect it's your gut flora that adapts; but it's not your DNA.

Why don't you go out and drink raw water for a while and report back after.

The question is whether drinking contaminated water has a survival advantage. It could have one if clean water is so scarce to the point that you're more likely to die of thirst before reproductive age than of bacterial infection.

Millenia is however a bloody long time to have daily diarrhea. But I commend them for trying. Good luck.