I'm not sure if this should be under ask science or ask historians, but uh, yeah, this confused me. You can't hydrate yourself with alcohol, it's just going to dehydrate you. So how would this have helped?
it's badhistory.
we will see if this causes /u/eternkerri to hulk smash
and here is the wiki section which debunks this
https://www.reddit.com/r/AskHistorians/wiki/health#wiki_drinking_water
Just to address your second comment, alcohol on it's own may be dehydrating, but diluted in beer or wine the water being ingested more than makes up for the effect.