Ethically it probably should be disclosed. Realistically, the defense can claim that they answered the questions based on the strict limitations of the question.
Did the house have any problems with the sewer lines? No. As explained, the sewer lines work properly. The pump station they are connected to doesn't.
Are you aware of any problems connected to.....sewere system of the property? No. Again, the problem is not the plumbing of the house, but the pumping station which is not part of the house. Although the pumping station loses power, the seller of the home wouldn't have any knowledge about its problems (unless he works there).
Plus, where is the damage? You said yourself that the city covered the damage from the overflow.