Download the Windows/Linux version and the version of Ren'Py Launcher that's used to create it (can be found by going into the game folder and reading script_version.txt. The version in question here is 7.4.11).
You can download the Ren'py Launcher here:
You must be registered to see the links
Use Betterzip or The Unarchiver to extract the game and then launch the Ren'py Launcher; make sure the launcher and game are in the same location or reconfigure the launcher in "preferences" (near the bottom) to the place you want it to search for games to convert.
Once the launcher is open, select the game on the right and then click on "Build distributions" on the left.
It'll scan the game first.
Then, on the next screen, select Macintosh in the Build Packages (unselect all other versions) and in Options below that make sure both "add from clauses to calls" and "force recompile" are selected, then press Build below that.
It'll create a zip file on a new folder, which you can then extract normally and play the game as a Mac application.