Probably, again, the disconect between common sense and science.
For the "common man" dehydration is not "Oooh, I feel a bit thirsty after run" it's "ooh, cripes I've got a headache and I feel faint."
In such a context, the calim makes sense, also, if I drink small amounts of water during physical labour that will prevent dehydration, won't it? Are you suggesting that, whilst working hard with my shirt off in the middle of summer having a canteen of water handy to take sips from is not a good idea?
You're over thinking this Tellos, really.
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