In case you had any doubts about the definition, Google Photos compresses photos if you select the ‘High Quality’ option, even for photos under 16 MP. What they mean is that photos above 16 MP will be visibly more blurry after compression than those under 16 MP. For photos under 16 MP, you will not be likely to notice the difference unless you zoom in using your computer or blow up the image to poster size.
In sum, if you do not want your photos to be touched at all, choose the Unlimited option. Google Photos do compress photos below 16 megapixels under the ‘High Quality’ mode.