Thats not PNG being lossy, the image was preprocessed to lose information by reducing color count before being compressed with PNG.
In PNG, you look for an image with 100 - 200 colors (so that colors repeat more often) and losslessly encode it with DEFLATE.
In JPG, you look for an image with with a reduced description through DCT, and losslessly encode it with Huffmann trees.
If color quantization using a specific algorithm was part of PNG you could say its lossy compression, its not, PNG is not lossy compression by any definition. Your specific image processing pipeline might be lossy, based on various tools you ran the image through or equipment used to capture, PNG is not.