AQ-modes are pretty much exactly FOR changing encoding to improve subjective quality at the expense of PSNR. Disabling AQ in --tune psnr would be entirely reasonable.
And I can’t think of any useful scenario for --tune psnr outside of PSNR benchmarking; it makes the video worse for any other purpose. Dark Shikari added that in x264 because she was annoyed at people claiming their (worse) H.264 encoders were better than x264 because of PSNR (a pretty lousy metric). So we got --tune psnr, as sort of middle finger saying “I can beat you at your stupid game too.”