So, if they had clarified your question as "How does gravity violate momentum with no power source" and responded with "Momentum is conserved with gravity, it's just that the effect of the column of water on the velocity of the Earth is negligible because of the difference in scale of scales between the two.", would you have been less confused?

Or did they also fail to explain the Newtonian "One type of potential energy is what you have due to height, and is equal to mass times height times acceleration due to gravity."? THAT would be a failure to teach Newtonian physics.