Hello. Although it is not possible to see the video to which the question refers, we can see, in the question, that water is removed from the cylinder as a gas is added to the same cylinder. So we can see that a gas was produced in the reaction, through the displacement of water, that is, if the water starts to be removed from the cylinder, it is because the reaction is producing a gas. If the gas is not produced, water will not come out of the cylinder.