I was always confused by this as well so just completely discounted the directions as something I did not have the facilities to easily measure because of the mix of weight and volume measures.
I would read that to mean 5 scoops of water to 1 scoop of Huel. This would mean 1050 ml of water per 3 scoops. That seems a lot thinner than I expect and would not even fit in the shaker bottle.