I was only trying to be helpful, it's not like I'm the person responsible for this game. But I have to admit that this is somewhat surprising to me. The game is using this:
ebitenutil.NewImageFromFile("assets/images/space_background.png")
If this code won't work on macOS, I think the biggest opportunity for improvement would lie on ebitenutil itself.
EDIT: (could it be related to missing the _ "image/png" import instead..? hmmm, most likely not, ebitenutil already makes that import)