I don't think anyone is truly arguing that medieval people never drank water. But they did drink beer in quantities that would be untenable today.
The author is also muddling points - medieval people didn't have to understand microbiology to know that beer was safer to drink.
And it was! Not only would the brewer have access to better water, it would be boiled as part of the process. (And aromatics like hops acted as mild antiseptics - the beer would be safe to drink for as long as it tasted well).
To review:
- It was tasty
- It was convenient
- People you knew who drink it had the trots less often
- It was cheap enough
- It made you feel good during long days of arduous labor
- There was no social stigma so long as you don't get drunk
And it makes plenty of reason that brewers would be incentivised to keep ABV low so people could drink it all day if they could.
Even going into the US prohibition, I think people would be astonished by how much the typical worker drank (usually cider in the US). With workplaces themselves providing it by the truckload.
Yes, people drank water. But (especially in urban settings) they drank A LOT of low abv drinks.