The dev uses webp format for the renders, so thats already a huge chunk of size less than it would be with png's.
Can it be compressed further? Always but you have to keep in mind that it is the dev's business and if he isn't happy with how it looks when it's further compressed, it's his choice. Sure a official compression is always better because a dev will most likely pay more attention to the quality, but like i said if thats the lowest he is willing to go you have to live with it and the unofficial compressed version.
Your reasoning that it would be barely visible is not the best argument to make since most people who frequent this site and play these games don't see the difference between low quality and high quality most of the time either way.