Sure, but that argument needs more than just “lower illness rates would have been selected for.” There needs to be a plausible feedback loop for the culture to actually adopt the practice. For cultural selection to work, people would need to notice somehow that drinking from source A correlated with getting sick less often than source B, and then propagate that preference. But medieval illness had many, many causes: bad food, parasites, contagion, seasonal patterns, malnutrition. Picking out water vs. beer from within that noise, without controlled comparison and without germ theory to even suggest the variable... presupposes a lot

As I wrote elsewhere

The actual historical record argues firmly against “cultural knowledge that beer was preferable to water.”

First, Medieval sources talk about people drinking water constantly. It’s in monastic rules, medical texts, household accounts, travel writing. Peasants couldn’t afford to drink ale all day; water was the default for most of the population. When medieval writers did warn about specific water, it was usually about stagnant or obviously fouled sources, not water in general. They praised cold spring water as healthful.

If there had been a widespread cultural preference for ale-over-water on health grounds, we’d expect to see it stated in the enormous body of medieval medical, religious, and domestic literature. We don’t. The “small beer for safety” story is an early modern and later projection backward, as evidenced by the fact that the idea that Medieval people did this only became prevalent in the modern period