No. PNG is lossless, it will do a really bad job at compression. Even those uniform colors won't actually be uniform because they are from image sensors.
JPEG is the appropriate comparison, as the authors are solving the same problem as JPEG tries to solve (i.e. a good enough lossy compression for natural images).
JPEG is the appropriate comparison, as the authors are solving the same problem as JPEG tries to solve (i.e. a good enough lossy compression for natural images).