I believe that it's because they don't own actually own a Mac to test it. And, because they have the option to make a Mac version with the engine that they are using, they thought that if they did it then it will most likely work because their windows export of the game worked. Because this site is filled with games made by one man teams, some people just don't have both platforms to test things. Hopefully you contacted the dev and tried to get it to work.
If anything, you can try and refund it.