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This response is quite ignorant from them. Effectively they are saying that microscopic scratches are within tolerances for their products unless you can prove that they cause issue.
However the really surprisingly ignorant response is the one about field flattening. A scratch is not an obstruction in the lens. A scratch will actually cause refraction of light. The effect scratches have on images will be more like ghosting, internal reflections, or other strange streaks around high contrast portions of an image. This is not something which can ever be corrected via a flat field image as the characteristics are dependent on the location and contrast of the light source. I.e. you may find it doesn't cause a problem at all when doing nebula but it may cause you lots of grief when doing open clusters.
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