Not 2 but 1/2. You did the division backwards and got the reciprocal. It is 0.5 tablespoons per person. What you got is 2 persons per tablespoon. Google states

Not 2 but 1/2. You did the division backwards and got the reciprocal. It is 0.5 tablespoons per person. What you got is 2 persons per tablespoon. Google states that for: "(4000 litres / 500000) in tablespoons": the result is "0.541024363 US tablespoons"