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Haha ~ interesting question
It didn't specified clearly in the video, but most of the time, people use rat as a model.
From wikipedia, it was 90 ml/kg in rats, which is similar to the value reported in the video, so I would guess it is a value from mouse model and translate it to human.
https://en.wikipedia.org/wiki/Water_intoxication

Make sense, but those poor mice :0

That might have been done during WW2...